The pH effect on microsphere formation in the dispersion copolymerization of methyl methacrylate with vinylbenzyl-terminated poly(acrylic acid) (PAA) macromonomers is studied. The diameter of poly(methyl methacrylate) (PMMA) microspheres is minimum around pH = 8. Dynamic light scattering data indicate that the shell thickness of PAA-grafted chains shows a maximum around this pH. The degree of expansion of PAA chains drastically affects the particle size of PMMA microspheres during the dispersion copolymerization. In previous work(1)), we prepared polymer microspheres by dispersion copolymerization of methyl methacrylate (MMA) with poly(methacrylic acid) (PMR) macromonomers in aqueous ethanol solution. The particle size (submicrometre to micrometre range) and size distribution are controlled by selecting the solvent composition and the feed amount and molecular weight of the PMA macromonomers. However, the particle diameter does not depend strongly on the pH of the solvent medium. More recently, we synthesized polymer microspheres by dispersion copolymerization of MMA with poly(acrylic acid) (PAA) macromonomers in aqueous ethanol solution(2)). The particle diameter of the polymer microspheres decreases as the water fraction in the mixed solvent, macromonomer concentration, and macromonomer molecular weight increased. For this dispersion copolymerization system it is expected that the pH in an aqueous solution drastically affects the particle size of the microspheres. In this short communication, we studied the pH effect in aqueous solution upon microsphere formation in the dispersion copolymerization of MMA with PAA macromonomers.