Block copolymers of styrene and methyl methacrylate were prepared by means of azo-benzoin initiators. In the first stage, prepolymers were synthesized by using these initiators. Photoactive prepolymers containing benzoin end groups were employed in the second stage for block copolymerization. The block copolymer structure was elucidated by means of extraction by appropriate solvents, GPC analysis, and IR and NMR spectral measurements. Mechanisms for initiation and possible termination for systems involving benzoin-type initiators are discussed in the light of results from studies of composition of the block copolymerization mixture. Structure and efficiency of the formation of the block copolymers depend greatly on both the relative reactivities of the photochemically derived radicals and the termination mode of polymerization.