UV irradiation of thin films of p-diethylaminobenzaldehyde diphenyl-hydrazone (1) produces 1-phenyl-3-(4-diethylamino-1-phenyl)-1,3-indazole (2) as the major photoproduct. A proposed mechanism for the reaction, which involves a photocyclization followed by an oxidation, is similar to the photocyclization of diphenylamine to carbazole and stilbene to phenanthrene. In order to gain more insight into the solid state photoreaction, and X-ray crystal structure analysis was performed on 1 to determine the geometry of the diphenylamine group relative to the hydrazone moiety. An X-ray structural analysis was also attempted for 2. In this case, however, owing to problems associated with crystal growth, we could establish the structure only qualitatively. © 1987.