The syntheses of several phenothiazine compounds bearing cationically polymerizable vinyl, vinyl ether and epoxide groups are described. These monomers were rapidly and efficiently polymerized or copolymerized by UV irradiation in the presence of onium salt photoinitiators. In addition, the monomers simultaneously function as efficient photosensitizers for the photolysis of the onium salts. In this article, the use of these polymerizable photos ensitizers for the photopolymerizations of epoxide and vinyl ether monomers is reported. The inclusion of a small amount of the monomeric photosensitizer is effective in markedly accelerating the cationic photopolymerization of the bulk monomer. Polymers prepared by the polymerization of the monomeric phenothiazine photos ensitizers also exhibit excellent photosensitization activity when used in combination with diaryliodonium, triarylsulfonium, and dialkylphenacylsulfonium salt photoinitiators.