Diphenylamine (DPA) is photocatalytically oxidized to N-phenyl-p-benzoquinone imine (PBQ) on the surface of CdO in ethanol under UV light as well as natural sunlight. The variation of the rate of PBQ formation on CdO surface with (i) [DPA] and (ii) airflow rate conforms to the Langmuir-Hinshelwood kinetic law. The catalyzed PBQ formation increases with the catalyst loading and light intensity. UV-C light is more effective in catalyzed PBQ formation than UV-A light, and the catalyst is reusable. The reaction mechanism on the illuminated CdO surface is discussed, and the kinetic constants deduced. V2O5 and ZnS enhance the photocatalyzed formation of PBQ.