The mechanisms of copolymerization of three anionic polymerizable surfactants (surfmers)-a methacrylic ester, a crotonic ester, and a maleic ester-in the emulsion terpolymerization of styrene/butyl acrylate/acrylic acid (S/BA/AA) were investigated. In order to clarify the contribution of each polymerization loci in the process, polymerization in the aqueous phase in the absence of both micelles and monomer droplets and emulsion polymerizations with different solids contents (30, 50, and 55 wt %) and different particle sizes were carried out. It was found that the methacrylic ester surfmer appeared to be extremely reactive and caused premature coagulation. The crotonic ester surfmer reacted only to a small extent and was not incorporated well enough into the polymer latex. The maleic diester surfmer appeared to have intermediate conversions varying between 50% and 95%. It was clearly shown that the degree of conversion of this surfmer depended on the particle diameter, with the conversion decreasing significantly at diameters higher than 100 nm. Although the maleate surfmer was the best of the three surfmers, its optimum use is not straightforward because high conversion of the surfmer led to the burying of a significant fraction of the maleate inside the polymer particles. Suggestions for a more advantageous incorporation are made.