Azobisisobutyronitrile, as a photosensitizer of the polymerization of N-vinylcarbazole in dichloromethane solution at 2 = 366nm, has also been shown to quench the polymerization that normally occurs in its absence. As the initiator concentration is raised, the polymer yields at first decrease and pass through a minimum before increasing to about twice the initial value, at the solubility limit of the initiator. The product polymers are predominantly formed through a cationic mechanism though a free radical fraction is also evident, particularly at high initiator concentrations. Kinetic mechanisms compatible with the experimental evidence are considered and indicate that the higher molecular weight cationic fraction is formed through the intermediacy of the first excited triplet states of both the monomer, and a complex of the monomer and solvent. Quenching of the former species by azobisisobutyronitrile results in initiation of the polymerization, but quenching of the latter species quenches polymerization. The low molecular weight radical fraction, on the other hand. is considered to be formed through the quenching of the first excited singlet state of the monomer by azobisisobutyronitrile.