To characterize the formation of H2O2 in photocatalytic reactions, HCHO and HCOOH have been photocatalytically decomposed by use of TiO2 and Pd/TiO2 under irradiation with the UV light emitted from a blacklight blue fluorescent lamp (BL) or germicidal lamp (GL). An enzymatic method has been introduced to accurately measure the H2O2 concentration. The photocatalyst irradiated with the UV light emitted from the GL produces H2O2 from water at a larger rate and amount. Deposition of I'd on TiO2 increases the rate and amount of the formation of H O, from water. In the absence of the photocatalyst, no HCOOH is decomposed under irradiations with both the UV lights from the BL and GL, whereas the decomposition becomes possible in the presence of the photocatalyst. H2O2 is formed in parallel with the photocatalytic decomposition of HCOOH. In the presence of the photocatalyst, the concentration of the H2O2 formed during the decomposition of HCHO is smaller than that formed from water alone. In contrast, this relationship is reversed with the decomposition of HCOOH. This is considered due to the formation of H(2)O2 in the process of the decomposition of HCOOH to carbon dioxide and water. The rates of decomposition of HCHO and HCOOH are directly proportional to their respective rates of formation of H2O2, indicating that there is a close relationship between the photocatalytic activity and the capability of the photocatalyst to form H2O2.