Oil-in-water microemulsions of mixtures of styrene and comonomer are easily prepared using titration methods in the presence of nonionic (NPn) or anionic (SDS) surfactants. Functionalized nanoparticles in the 20-30-nm diameter range bearing chloromethyl, active-ester, acid or pyridyl surface end-groups are prepared by polymerization of microemulsions containing mixture of styrene (St) and, respectively, vinylbenzylchloride (VBC), N-acryloyloxysuccinimide (NHA), methacrylic acid (MA) or vinylpyridine (VP). Reactions of nucleophiles on particles bearing either chloromethyl or active-eater surface end-groups, performed directly in the aqueous suspensions, give rise to a wide range of nanoparticles with various functionalities. The main role of the surfactant on such surface reactions is demonstrated and used to improve the reaction yields.