Platinized CaTiO3 powder (band gap 3.5 eV) had a high photocatalytic activity of decomposing posing water into H-2 and O-2 under the irradiation of UV light, especially with photon energies above 3.8 eV. The existence of this photocatalytic activity is further supported on electrochemical grounds in that the photoinduced current spectrum measured between a CaTiO3 single crystal and a Pt electrode without applied voltage in water showed a maximum near 4.1 eV This result indicates that the direct measurement of the spectrum corresponding to the efficiency of water decomposition is an effective method to survey photocatalytic activities of materials. (C) 2002 Elsevier Science Ltd. All rights reserved.