Monodisperse carboxylated micrometer-sized polystyrene particles were synthesized by dispersion polymerization of styrene in ethanol and in 95% ethanol-water in the presence of acrylic acid ( AA) as a functional comonomer. When AA was present at the onset of the reaction, the resulting particles had an increased particle size and a broader size distribution than those prepared in the absence of AA. If, however, the addition of AA was delayed ca. 1 h, so that the particle nucleation stage was complete, then 2 wt % AA, dissolved in monomer plus solvent, could be added to the reaction without a deleterious effect on particle formation. When larger amounts of AA ( e. g., 4 wt % based on styrene) were added, most resulting particles became unstable and coagulated if PVP55 (M-w approximate to 55000) was used as the stabilizer. Some evidence points to interaction between - COOH groups on the particles as the origin of particle aggregation. Improved colloidal stability was achieved by decreasing the solids content, increasing the polarity of the medium, and switching to PVP360 (M-w approximate to 360000) as the polymeric stabilizer. Coagulum-free particles with a very narrow size distribution containing 2%, 4%, and 6 wt % AA could be obtained in this way. Under these conditions, adding different amounts of AA in the particle growth stage did not change the particle size or size distribution.