The soapless emulsion copolymerization of methyl methacrylate (MMA) and n-butyl acrylate (n-BuA) at four levels of monomer feed composition (f10) was studied. Conversion (X), average particle diameter (Dp), molecular weight distribution (MWD), surface charge density, and glass transition temperature (Tg) of the copolymer as a function of reaction time (t) were measured. The copolymers obtained even at low conversion, except for the run of (f10) = 90 wt. percent MMA, exhibit two Tgs in their DSC thermograms. Phase separation is found to occur in the latex particles during polymerization. The heterogeneous distribution of monomers in particles, in which a relatively rich MMA region exists in the shell and a relatively rich n-BuA region exists in the core of the particles, is assumed to arise from phase separation. The average copolymer composition and the fraction of the two domains are estimated. The polymerizatrion course and particle size growth follow the linear X vs. t2 and Dp3/2 vs. t relationships, respectively. Although the coagulation of particles happens after around 30 percent conversion, the polymerization behaviors, except for increasing rates, are not affected.