The photocyclization–oxidation reaction of 4-stilbenecarboxaldehyde gave 3-phenanthrenecarboxaldehyde. Similarly, irradiation of 3-stilbenecarboxaldehyde gave a mixture of 2-and 4-phenanthrenecarboxaldehyde. Irradiation of 1-(p-formylphenyl)-4-phenyl-1,3-butadiene gave a mixture of 1-(p-formylphenyl)naphthalene and 8-phenyl-2-naphthaldehyde. Photocyclization of 1-(m-formylphenyl)-4-phenyl-1,3-butadiene yielded 1-(m-formylphenyl)naphthalene, 5-phenyl-2-naphthaldehyde, and 8-phenyl-1-naphthaldehyde. The surprising nature of the photocyclization–oxidation reactions of stilbenes and 1,4-diphenyl-1,3-butadienes containing formyl substituents is discussed.