Monodisperse styrene/methyl methacrylate/acrylic acid (St/MMA/AA) copolymer microspheres have been prepared with surfactant-free emulsion polymerization in air. The presence of oxygen in the system not only caused an induction period but also decreased the average particle size (D-p). However increasing AA concentration ([AA]) gave a reduction in the induction period. The FTIR and NMR analysis of the latex copolymer confirmed that the correlation of the copolymer compositions with the feed compositions was much better at the lower [AA] than at the higher levels. The AA contents of the copolymers obtained in air were much lower than those of the copolymers obtained under N-2 protection. Decreasing [AA] led to decrease in the copolymer molecular weight and broadening of the molecular weight distribution, but the particle size distribution (delta/D-p) was unaffected. In addition, the average particle diameter (D-p) was proportional to [AA](-0-2.55), and increasing comonomers feed content caused linear increase of D-p, and a monodisperse sample with final solids contents up to 34.2 wt % was obtained. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1598-1605, 2009