Reactive microgels (RMs) were synthesized by a self-emulsifying copolymerization of an unsaturated polyester (UP) and styrene (St) with azobis(isobutyronitrile) (AIBN) as initiator. The influence of UP/St mass ratio on copolymerization rate, composition, molar mass (M(w)), particle diameter (d(z)), intrinsic viscosity ([eta]), polydispersity index (M(w)/M(n)), and glass transition temperature (T-g) of RMs has been investigated. In agreement with an earlier experiment where an electrolyte (KCl) was added to the reaction mixture, the composition of the RM did not change with increasing conversion. It must be concluded therefore that the presence of this electrolyte has no influence on the composition of the RM. The constant composition implies that practically no diffusion of the reaction components to growing RM particles takes place during the copolymerization.