Factors regulating the formation of gaseous H2O2 and O3, which are considered to be dominant oxidants for SO2 in the atmosphere, were studied in Los Angeles and its vicinity. The concentration of gaseous H2O2, as well as O3, was highest in the early afternoon at all sites studied when the sky was clear or partially cloudy, whereas when it was hazy and foggy, H2O2 concentrations were substantially lower than those under clear or partially cloudy conditions. To study the seasonal variation of gaseous H2O2, its concentration was measured in west Los Angeles in the early afternoon (1200--1600 LT) during August 1985 to September 1988 as well as at other souther California sites. The observed total range of gaseous H2O2 concentration was 0.03--1.35 (n=150), 0.20--2.04 (n=16), and 0.43--1.72 (n=13) ppb at Los Angeles, the Mojave Desert, and the San Bernardino Mountains, respectively. The mean seasonal H2O2 concentrations at Westwood, Los Angeles, were 1 ppb in summer and approximately 0.2 ppb in winter. The O3 concentration during each H2O2 measurement period ranged from 10 to 230, 48 to 95, and 120 to 160 ppb for the same sampling locations, and seasonal variation was not as pronounced as for H2O2. Statistical analysis indicated that (1) H2O2 was most correlated with solar radiation, whereas O3 was most correlated with temperature, the height of inversion layer, and the concentration of NOx, and (2) H2O2 concentration was inversely correlated with SO2 during warm months of 1987--1988, when haze events were frequently observed. From our observations we deduce that in Los Angeles air (1) the production rate of sulfuric acid from SO2 is significantly lower in winter than summer due to lack of sufficient amounts of oxidants, and (2) thus the reduction of SO2 emission from anthropogenic sources, which has been accomplished by electric utilities, has not resulted in the equivalent reduction in sulfuric acid in rainwater, where rain events principally occur in winter. ¿ American Geophysical Union 1989