They call it the paillasson pure indulgence — a holistic spa treatment that involves a coconut-husk scrub (paillasson) followed by a coconut-oil massage done at the Oberoi Spa by Banyan Tree at the posh Oberoi Resort in Mauritius. The dark-brown husk glides across my back, exfoliating my skin before the masseuse performs magic on my road-weary muscles. Coconut is the overwhelming smell, but every so often I detect exotic odors that my nose can't readily identify. The masseuse tells me these aromas are ingredients for other spa treatments being prepared in a nearby room — potions made from ylang-ylang, coriander, jasmine and tamarind.

Hoping to learn more about the natural ingredients they use in spa treatments, I attend the resort's weekly aromatherapy workshop inside its wine cellar. About a dozen of us gather around a huge wooden table covered in petals and leaves, as well as vials of essential oils.

Workshop leader Anil Jhurry takes us on a journey through the history of scent, gradually narrowing his focus to the Indian Ocean islands of Mauritius and Réunion, where the raw ingredients for many of the world's luxury perfumes and aromatherapy treatments are cultivated: exotic plants like vetiver and girofle (clove buds) as well as fl ora like vanilla, geranium and ylang-ylang. Although little of this sweet-smelling fl ora is native to the southwest corner of the Indian Ocean, Jhurry explains that the tropical trade wind climate and volcanic soil are ideal for cultivating these plants. It also helped that the French — the world's paramount perfume makers — had a profound colonial presence on both islands.

Passing a vial of ylang-ylang oil beneath my nose, I have to admit that it smells wonderful, a rich floral aroma that is most defi nitely not your grandmother's perfume. But in the same breath, I fi nd myself wondering what the big deal is. Lots of things smell nice. "What makes this so special?" I ask.

"Beyond simply smelling good," Jhurry responds, "many of these plants provide a variety of therapeutic benefits." Ylang-ylang, it seems, is extremely effective for calming and bringing about a sense of relaxation. But it is also an aphrodisiac.

During the workshop, we make potpourri from the ingredients on the tabletop. Reaching into a huge bowl in the middle of the table, I grab a handful of dried frangipani leaves and drop them into my ceramic bowl. By now everyone is sniffi ng vials of essential oils and studiously trying to decide which combination they should sprinkle on their leaves. I experiment with lavender and vanilla, shake the whole thing vigorously and take a whiff. Phew! It nearly knocks me out. Jhurry laughs and says that I've used way too much lavender and suggests that I "sweeten" my potpourri by adding more tiny drops of vanilla oil. Eventually I get it right, dump everything into a cheesecloth sack and — voilà! I've got a custom-made aromatic souvenir of my visit to Mauritius.

My interface with local aromas could have ended right there. But the combination of my spa experience and aroma workshop sparked my curiosity. I could have easily whiled away my days on the beach, but I was game for something new — an olfactory journey across Mauritius and nearby Réunion, both of them off the east coast of Madagascar. I wanted to fi nd out where these plants are grown and how they start their long and winding road from the Indian Ocean to the perfumes that scent the necks of millions of women.

Ylang-ylang became a star more than 80 years ago when several Parisian fashion houses used it as a key ingredient in perfumes such as Chanel N°5 (which launched in 1921) and Jean Patou's ultra-expensive Joy (1930). It was vanilla oil from Réunion that made Shalimar such a hit when it was introduced by Guerlain in 1925. And the trend of using Indian Ocean ingredients (especially ylang-ylang) continues today in funky post-modern perfumes like Sui Dreams by Anna Sui (ylang-ylang) and Revlon's Charlie Blue (geranium).

The aromatic path takes me through endless sugar-cane fields and around jagged volcanic peaks to the swish Mövenpick Resort & Spa, where I come across Whiff! It's a sophisticated atelier du parfum located in a breezy arcade overlooking the Indian Ocean, where you can transform your favorite scents and essential oils into a custommade perfume. You bottle it on the spot and slap it with your very own label. Call it what you will — Eau d' Colin or Janelle N°5 — it is even more unique than having your own wine label.

Soon after entering the shop, I learn what it means to bottle at the source. The whole purpose of Whiff! is to devise your own scent. But I'm having a hard time deciding what's right for me. Laikrany Seehakoo, a shop assistant, explains the process: "You choose three notes — hot, base and head — and then blend them together. Men seem to prefer stronger scents like bergamot and fern. But choose anything that suits you." Bergamot has a defi nite citrus scent and, in fact, it's made from the peel of a certain type of orange. Fern has a woodsy aroma.

Laikrany drips some essential oil on her wrist and holds it up to my nose. "This is my favorite," she croons. "Vetiver — a small fl ower that grows near the sugar-cane fields. We use it a lot in Hindu ceremonies, especially weddings, but it's very rare these days." Its rarity may be due to the fact that there are a lot of Hindu ceremonies on Mauritius, a one-time French and British colonial possession that is now largely populated by Indians and Chinese.

My final brew contains a mixedcitrus note called "coral," blended with star fruit and oriental musk — an aroma that's both refi ned and a little on the wild side. I toy with various names before deciding on Eau de Control, which seems to summarize both my life and my slapdash perfume making.

To explore the other half of this aromatic equation, nearby Réunion, I take a 45-minute fl ight to the island, rent a zippy little French sedan at the airport and head down the east coast into the heart of the vanilla-growing region, a prime crop along with geraniums.

It's a steamy tropical morning when I arrive at the Coopérative de Vanille in the village of Bras-Panon. Vanilla has long been the "king crop" on this French island, thanks to a 12-year-old slave boy from Réunion by the name of Edmond Albius who in 1841 perfected a simple way to pollinate vanilla fl owers artificially. This made commercial cultivation possible for the fi rst time and spurred the island's vanilla boom. Almost two centuries later, vanilla plantations are among Réunion's more popular sights.

"The temperature and humidity on the east side of Réunion are ideal for vanilla," says co-op manager Francois Mayer as we stroll through an old warehouse now used for historical exhibits and vanilla-processing demonstrations. "Réunion doesn't produce an awful lot of vanilla compared to places like Madagascar and the Comoros, but we like to think that we grow the best," says Mayer. "And the savoir-faire, the know-how, comes from Réunion. From here, it spread to the rest of the world."

While most of the vanilla produced on Réunion is exported to France, the islanders do keep some for themselves, using it mainly in cooking. Among typical dishes that I come across in Creole restaurants on the island are chicken or duck à la vanilla, as well as incredibly tasty vanilla ice cream, crêpes, rum punch and — my favorite — crème brûlée flavored with fresh vanilla, which I eat nearly every morning, noon and night.

Although known as a decorative flower to the rest of the world, a type of rose-scented geranium (Pelargonium graveolens) called the bourbon geranium thrives on Réunion and is well-suited for perfume, soaps and aromatherapy because it blends well with other essential oils like clove, lavender and sandalwood. And though there have been attempts to cultivate it elsewhere, nobody has been able to replicate the bourbon's distinctive aroma — a bit like traditional rose oil but with a touch of mint.

On Réunion's west coast, I discover white-sand strands that offer a refreshing respite from the humid clime. Volcanoes rise steeply behind the coast, their upper reaches smothered in pine plantations and geranium fields that glisten with yellow-green fl owers during the growing season. The production area — which has the perfect soil and humidity for growing geraniums — lies at around 3,000 feet. Reaching it takes a bit of effort: an hour-long drive on switchbacks up the fl ank of a huge volcanic caldera called the Cirque de Mafate. Just past the village of Petite France, I spy La Distillerie du Maïdo, a geranium plantation nestled among the pines where I amble through the harvested fi elds, learn about the distillation process that turns the petals into essential oil and browse a plantation shop of soaps, massage oils and candles made from geranium and other local plant products.

"The geranium region stretches for about fi ve kilometers around here," says Olivier Favre, the hip, young shop manager and son of the plantation owner. "We harvest in the winter — June, July and August [in the southern hemisphere]."

Whiling away an afternoon in one of the sidewalk cafés of St.-Denis, Réunion's Frenchcolonial capital on the north coast, I catch a whiff of something familiar and look up. A woman flounces by in a miniskirt and high heels. I can't see her face, but her scent is unmistakable: ylang-ylang perfume made from the delicate yellow fl ower, the aphrodisiac. My nose isn't yet good enough to say which one — Opium, Obsession, or one of the others. But I now recognize it as a quintessential aroma of the Indian Ocean.