Ibuprofen-loaded polystyrene microparticles were prepared by the emulsion-solvent evaporation method from an aqueous system containing methylcellulose as the emulsion stabilizer. The effect of production variables on the formation and the physical characteristics of the microparticles were studied. While an increase in stirring speed, concentration of emulsion stabilizer up to a critical value and disperse phase volume decreased the mean diameter of the particles, increase in drug loading and concentrations of emulsion stabilizer beyond the critical value tended to increase in size. The microparticles, except those prepared at higher stirring speeds, and with higher disperse phase volumes and drug-loads, were found to be spherical.