The factors for controlling the photochemical generation of gaseous H2O2 were studied on the basis of both field data and model predictions. Field data of the concentration of gaseous H2O2 at ground-level were obtained in west Los Angeles during 1985--88. The results indicate that there is a clear seasonal trend of gaseous H2O2 (0.03--1.35 ppb, n=150) with highest concentrations in the summer. Statistical analyses revealed that the concentration of H2O2 increases with increasing solar radiation at a statistically significant level. A one-dimensional photochemical model, which is based on the prevailing meteorological conditions and primary pollutants loading in Los Angeles, also predicted that solar radiation is the major factor in determining the formation rate of H2O2 in the atmosphere. These results indicate that solar radiation significantly controls the formation of gaseous H2O2. Considering the low photochemical activity in the winter, H2O2 is probably limiting for the oxidation of SO2 in the Los Angeles winter atmosphere where the SO2 content exceeds the H2O2 concentrations. Our results also imply that, if the UV radiation in the troposphere increases due to the depletion of the stratospheric O3 content, the H2O2 levels will increase and as a result the production of sulfuric acid will increase in the winter atmosphere. ¿ American Geophysical Union 1990