Copper(II) oxide (CuO)-modified indium oxide (In2O3) prepared by physical mixing efficiently mineralized acetone under irradiation of light from a blue LED. The reaction rate increased with additional irradiation of light from a red LED, though no reactions occurred over CuO and In2O3 under irradiation of the red light, indicating that band-gap excitation of CuO contributed to photocatalytic reaction over CuO-In2O3. The effect of CuO modification was also observed in action spectra with and without irradiation of the red light.