Poly(p-nitrophenylacrylate-co-methaerylamide) and poly(p-Nitrophenylacrylate-co-N,N'-isopropylacrylamide) reactive microgels were synthesized by precipitation polymerization. The process was followed qualitatively by infrared spectroscopy (ATR-FTIR) and microgels composition was determined by nuclear magnetic resonance (H-1 NMR). Scanning electron microscopy of obtained colloidal particles showed strictly spherical morphologies with a moderate polydispersity. The average hydrodynamic particle diameter and particle size distributions were measured by quasi-elastic light scattering and the particle size distributions obtained ranged from 100 to 600 mn. Several synthetic parameters affect the particle size of these materials and thus, indirectly, their properties and future applications. In this article, we report the influence of different polymerization reaction conditions in the final microgel dimensions. For example, we observed that the different solvent-comonomer affinity induced a significant change in swollen particle size of the copolymeric microgels. On the other hand, the crosslinking density limited the particle sizes, but an excess of crosslinker content in the reaction mixture resulted in the opposite effect. Finally, we also studied the influence of initiator content in the mean particle size. (C) 2007 Wiley Periodicals, Inc.