Photocatalytic activity of KTaO 3 catalyst to water splitting is much improved by organic dye-modification. In particular, cyanocobalamin (Vitamin B 12 ) is the most effective for improving water splitting activity, and the formation rate of H 2 and O 2 are achieved values of 1912 and 979 μimol g cat“ ’h” 1 , respectively, under the light irradiation from Xe lamp (300W). Mechanism of photocatalytic H 2 O splitting into H 2 and O 2 on PtO x /KTa(Zr)O 3 modified with various dyes were investigated. Photoluminescence (PL) spectra of KTaO 3 catalysts suggested that the excitation energy transfers from KTaO 3 to Cr-Tetraphenylporphyrin (Cr-TPP) and PtO x co-catalyst. Transient response of photovoltaic potential revealed that the excitation state of KTaO 3 was enlarged over 500 μs by dye-modification. Photo-excited electron in KTaO 3 transfers to dye, and a long charge separation state was achieved. Thus, it was suggested that the photocatalytic activity was much improved by increasing lifetime of photo-excited charges which can reach to the reaction site effectively.