Thin films of silver salt of alginic acid, a typical high molecular carboxylic acid in nature, were photolyzed by 253.7 nm light. On irradiating with a 15‐W sterilization lamp in air at relative humidity of more than 70%, the silver alginate films first became yellow‐brown colored due to formation of photolytic colloidal silver particles. When irradiation was continued, the irradiated surface of the films finally changed into clear silver mirror. The morphology of these films was observed by means of a high‐resolution scanning electron microscope. Colloidal silver particles (10–50 nm diam) formed by a short‐time irradiation were sparsely distributed at the film surface. As a result of prolonged irradiation for ∼180 min, film surface was covered with aggregated colloidal silver. The x‐ray diffraction study of the irradiated films revealed sharp diffraction lines, indicating that the colloidal silver was in a highly crystalline state. A preliminary observation of a microtomed cross section of the film showed that colloidal silver particles had a tendency to precipitate at the irradiated side of the film. These observations were consistent with more than a 108‐fold decrease of the sheet resistance and change in transmittance spectra of the films caused by photolysis. It has been proposed that silver atoms from the silver alginate migrate and coalesce to yield the colloidal silver. The colloidal silver particles further diffuse in the film and aggregate themselves at the irradiated surface, giving rise to silver metal films.