In order to explore the possibility of applying ozone to the treatment of photographic waste water, the change in COD during the oxidative decomposition by ozone was investigated for some of the most widely-used photo-processing chemicals. Developing agents, sodium sulfite and thiosulf ate showed relatively high rate of decomposition, however, complete decomposition was achieved only in sulfite and in dilute thiosulf ate. Benzylalcohol, as well as acetic acid, were not decomposed, whereas cliethyleneglycol and formbisulfite showed the increase in COD with the progress of oxidation. The results indicate that, though ozone is a mighty oxidizing agent, it cannot afford the complete reduction of COD for most of photo-processing waste waters, unless some other technologies are combined with it.