The sensitivity of light-sensitive material depends both on the quantum yield of the photochemical reaction and on the property change (e.g., absorbance, solubility, and so on) which is brought about by the reaction. The sensitivity of photopolymers of the photopolymerization type is improved by increasing the latter factor. Improvements of photopolymers of the photocrosslinking type may be possible by increasing the former factor.A photosensitive ionomer, a polymer which on exposure crosslinks through metallic ions of variable valence, was prepared. A photosensitive plate coated with the ionomer formed a visible and relief image on irradiation, and the latent image was intensified by treating with a reducing or a oxidizing developer.Mercurous copoly(vinyl alcohol–maleate) was prepared as an example of a photosensitive ionomer and the image-forming properties were compared with those of mercurous copoly(vinyl alcohol–acrylate) and mercurous copoly(vinyl alcohol–methacrylate) which were previously reported. Mercurous copoly(vinyl alcohol–maleate) was found to be the most sensitive for forming a visible image, but the least sensitive for a relief image. It was considered that the structure of this ionomer, in which two mercurous carboxylate groups were adjacent, was suitable to the photochemical reaction, but the reaction did not bring about enough property change to form a relief image.