Polymerization rates in SDS/monomer/water oil-in-water microemulsions were measured employing a water-soluble (2,2 -azobis(2-amidinopropano), ABAP) and an oil-soluble (2,2 -azobis(isobutyronitrile), AIBN) photoinitiators. Monomers of widely different hydrophobicity, ranging from acrylamide and methacrylic acid to ethylhexyl methacrylate (EHMA) and styrene, were employed in order to evaluate the role of the monomer distribution between the aqueous and microaggregate pseudophases on the photoinitiation efficiency. ABAP was more efficient initiator, irrespective of the monomer distribution. The lower efficiency of AIBN was explained in terms of intracage geminate recombination. The initiation efficiency of AIBN relative to ABAP varied from 0.35 (EHMA) to 0.02 (styrene). The high value for EHMA indicates that the initiation is due to radicals generated in the microaggegates, and that nearly one third of radicals avoids recombination in the original supercage. The low value of styrene is explained in terms of a fast addition to the monomer that reduces the radical exit rate