Functionalized silicone copolymers containing thioxanthone side groups were prepared and evaluated as radical polymerization photoinitiators in the presence of 4-(dimethylamino)benzoate ester derivatives as coinitiators and acrylic esters as monomers. Compared to low molecular weight thioxanthone analogues, the functionalized polysiloxanes exhibit a polymer effect similar to that observed with polysiloxanes containing benzophenone side groups : the photoinitiating efficiency is enhanced when thioxanthone chromophores are present as pendant groups on polymer chains and when the tertiary amine is a free reactant in the medium. This effect disappears when the chromophores and the amino groups are attached to the same chains. Time-resolved spectroscopy was used to compare the quenching of thioxanthone triplets in several systems where the-ketone and the amine reactants are present in the form of pendant groups in copolymers or as low molecular weight reactants. The photophysical study performed in media different in their nature and viscosity indicates that the polymer effect is not to be found in the efficiency of the primary deactivation process.