Water-insoluble comonomers were found to impart diffusional stability to the miniemulsion droplets. These droplets became the principal site of nucleation and copolymerization. The resulting copolymer composition tended to follow what was predicted by the copolymer equation, and was governed by the reactivity ratios. In contrast copolymer compositions obtained from macroemulsion copolymerizations tended to be more influenced by the relative water-solubility of the comonomer and mass transfer.