The ultrasonically initiated batch miniemulsion copolymerizations of methyl methacrylate (MMA) and butyl acrylate (BA) are studied at different MMA:BA ratios (including homopolymerizations), and the physical properties and chemical composition of the polymers formed are investigated. Trends in the evolution of the particle number are rationalized with reference to the mechanical properties of the polymer particles, with the number concentration of the softer, BA-rich particles reducing with continued sonication. Molecular weight data are consistent with high radical fluxes entering the particles, with the radical entry frequency calculated from the peroxide yield in a model system to be similar to 1.5 x 10(-2) s(-1). To within experimental uncertainty, the copolymer composition is found to be consistent with the terminal model for propagation reactions and previously published reactivity ratios; hence, it is concluded that ultrasound has little effect on the propagation step in a free-radical polymerization process. The results obtained also support a miniemulsion polymerization pathway for sonochemically synthesized latex particles.