A study of the photoinduced cationic polymerization of a novel series of cyclic and acyclic ketene acetal monomers was carried out. It was observed that the cyclic monomers underwent facile cationic polymerization to give high molecular weight polymers while the acyclic ketene acetals did not. Thermally induced polymerization was also observed on standing catalyzed by glass surfaces. Photoinduced cationic polymerizations employing both diaryliodonium and triarylsulfonium salt photoinitiators were studied. It was observed that the position and type of substitutents in the cyclic ketene acetals was the major factor in determining the proportion of vinyl and ring-opening polymerizations that take place.