The effect of amino acids on the photocatalytic toxicity of titanium dioxide (TiO₂) was investigated. Various amino acids were treated with ultraviolet irradiation along with rutile and anatase types of TiO₂ particles. The photooxidized amino acids, especially cysteine, generated hydrogen peroxide in the presence of copper ions during the auto-oxidation process. In the case of aromatic amino acids, hydrogen peroxide formation from photocatalyzed phenylalanine was significantly larger than that from tyrosine. Hydrogen peroxide is a long-lived reactive oxygen species. Furthermore, photocatalyzed amino acids gradually generate hydrogen peroxide. This secondary generation of hydrogen peroxide may play an important role in the cytotoxicity of the TiO₂ photocatalyst after photoirradiation.