The evolution of the reaction conversion and particle size distribution during the microemulsion polymerization of methyl methacrylate (MMA) is used to determine the particle nucleation mechanisms. A pseudo-3-component oil-in-water microemulsion is formed with water, MMA, and a mixture of dodecyltrimethylammonium bromide (DTAB) and diodecyldimethylammonium bromide (DDAB) in a 3:1 weight ratio as surfactant. Polymerization is initiated with either an oil-soluble or a water-soluble initiator and conversion followed either by measurement of the unpolymerized monomer concentration in samples taken during the reaction or by direct on-line densimetry. A two-stage process is observed. The first stage, described by a very slow increase in conversion, is attributed mainly to homogeneous nucleation, and the second stage, characterized by a much higher rate of conversion, involves continuous nucleation and is governed mainly by a micellar-entry mechanism.