By utilizing a batch-recycle photoreaction system, decomposition characteristics of formic acid and hydrogen peroxide are precisely examined in the course of 254-nm light irradiation of dilute, aqueous formic acid solution added with hydrogen peroxide. Irrespective of the initial concentration level of hydrogen peroxide, its decreasing rate is low, while formic acid remains in the presense of oxygen. The decomposition rate of formic acid is proportional to the irradiation intensity and to the 0.5 th order of hydrogen peroxide concentration. The effect of formic acid concentration is low. The observed results are simulated with a photoreaction model which incorporates a group of elementary steps. The reaction rate constant of the photo-initiation step is firstly determined for the autophotolysis of hydrogen peroxide. To evaluate the oxidative decomposition kinetics of formic acid, a model is then developed which includes subtraction of the hydrogen atom from formic acid molecule by a hydroxyl radical and reproduction of hydrogen peroxide through addition of oxygen to the formic acid derived radical. The present reaction model simulates the decomposition characteristics of formic acid and the unique concentration change of hydrogen peroxide fairly well.