Photooxygenation of cinnamic acid derivatives was investigated by using titanium dioxide as a heterogeneous photocatalyst. Benzaldehyde derivatives were efficiently produced by the photooxygenation of the cinnamic acid derivatives having methoxy groups on the phenyl ring. When titanium dioxide was added to acetonitrile solutions of methoxy-substituted cinnamic acids, the color of the suspension changed from colorless to pale yellow. The photooxygenation might proceed through photoinduced single electron transfer from cinnamic acid derivatives to titanium dioxide. Adsorption of the carboxyl group of cinnamic acid derivatives onto the surface of titanium dioxide accelerated the photoinduced electron transfer.