Reactive radicals such as hydroxyl radical (OH.), hydrogen peroxide (H2O2), and superoxide anion (O-2(-)) are the main power driving titanium dioxide (TiO2) photocatalytic reactions, for example, photokilling of biological cells. Here, the effect of copper ions on the formation of H2O2 over photocatalytic TiO2 was investigated. Under an oxygen-purged solution, formation of H2O2 was increased dramatically up to 20 times by the addition of a small amount of copper ions. By using the Fenton reaction, the H2O2 formed can be converted into OH. a highly reactive radical. In contrast, under nitrogen-purged solution, no H2O2 was formed even in the presence of an electron acceptor, silver ion (Ag+). These results clearly show that H2O2 was generated from the reduction site of the photoexcited TiO2 and indicate an effective way to increase the photocatalytic efficiency. (C) 2003 Elsevier Inc. All rights reserved.