Narrowly distributed nanoparticles of poly (n-butyl methacrylate-co-vinyl pyrrolidone) were prepared through microemulsion polymerization with a nonionic surfactant of Tween-80 as emulsifier (6 wt % of the latex) and n-butanol as coemulsifier. The polymerizations were initiated with benzoylperoxide (BPO), potassium persulfate (KPS), KPS/ferric sulfate (FeSO4), and BPO/FeSO4, respectively, where the initiation in the case of BPO/FeSO4 took place mainly at the interphase between the oil phase and the reaction media. Namely, this interfacial-initiated microemulsion polymerization resulted in larger particles with relatively narrower particle size distribution as well as higher limiting monomer conversion but lower polymerization rate compared with the polymerization initiated with KPS/FeSO4. In this article, the influences of initiation method, monomer ratio, and addition of water-soluble components on microemulsion polymerization and latex particle size were studied to discuss the mechanism of interfacial-initiated microemulsion polymerization. (C) 2004 Wiley Periodicals, Inc.