We recently found that photocatalytic reactions produce H2O2 at a concentration level of ppm (parts per million) from water. For some reason, however, other researchers have paid little attention to the formation of H2O2 in this reaction. Of course, one of the reasons is due to low sensitivities of the conventional methods employed to measure the H2O2 concentration. However, we suppose that there is another reason which makes it difficult to measure it. To elucidate this reason, therefore, an effect of linear liquid velocity on the photocatalytic formation of H2O2 from water is experimentally investigated, and an explanation for the experimental result is made using a mathematical model that takes into consideration the formation and decomposition of H2O2 and its diffusion through a film in the neighborhood of the photocatalyst surface into a bulk liquid. This effect is also discussed on the formation of H2O2 from an aqueous solution of formic acid. As a result, it is concluded that the diffusion film is responsible for the reduction in the H2O2 concentration.