The polymerization of styrene-in-water microemulsions prepared using polymerizable cosurfactants, such as hydroxyalkyl acrylates or methacrylates, gives rise to stable suspensions of well-defined highly functionalized nanoparticles in the 15-25 nm diameter range. Among the various associations of ionic surfactants with hydroxyalkyl (meth)acrylate cosurfactants tested, the surfactant/ cosurfactant system (sodium dodecyl sulfate/2-hydroxypropyl methacrylate) is found to be the most effective and allows the preparation of oil in water microemulsions of styrene (up to 7 wt %) or mixture of monomers. Polymerization with 100% conversion is achieved at room temperature using either an oil-soluble photoinitiator (DMPA) or water-soluble redox systems (hydrogen peroxide/ascorbic acid or ammonium persulfate/diamine). GC monitoring of conversion for both monomers clearly demonstrates that the mechanism of copolymerization depends on the location of the reactive species(monomer, polymerizable cosurfactant, and radical initiator). The resulting polymers contain up to 40% of hydroxylated units.