Miniemulsion polymerization reactions of methyl methacrylate (MMA) and vinyl acetate (VAc) for the synthesis of biocompatible polymeric nanoparticles were comparatively studied. 2,2'-Azo-bis(isobutyronitrile) was used as initiator, lecithin as surfactant, and Miglyol 812 or castor oil as costabilizers. Despite the propagation coefficient of VAc being ten times higher than that of MMA, the reaction rates of the VAc polymerizations were much lower. In VAc polymerizations a higher amount of low-molar-mass polymer was formed which may be attributed to transfer reactions to low-molar-mass species. Polymerizations with a high Miglyol 812/VAc ratio were slower and resulted in the formation of a higher amount of low-molar-mass polymer compared to polymerizations with a low Miglyol 812/VAc ratio or VAc miniemulsion polymerizations with hexadecane as costabilizer. In reactions with high Miglyol 812/VAc ratios the peak area of the GPC trace corresponding to the polymer was substantially higher than the theoretical polymer content.