Symmetrical azo compounds of different charge were used as photoinitiators of methyl methacrylate (MMA) polymerization in homogeneous solution and in microemulsions supported by cationic and anionic surfactants. The measured constants for association of initiators to the aggregates are those expected from the micellar surface potential, which is reduced by the presence of monomer. Polymerization rates and photocleavage yields of azo compounds were measured in different media. The results are interpreted in terms of a fast interchange of the counterions between micelles that reduces the intra-Stern layer primary radical combination.