The historic trend of the levels of gaseous H2O2 during 1960's--1980's in Los Angeles was assessed from field data and numerical simulations. The results indicate that during 1960's--1970's concentrations of H2O2, as well as O3, were likely to have been higher than those during 1980's. For the period 1969--1987, predicted summer peak concentrations of H2O2 and O3 in the ground level atmosphere were in a range of 6--10 ppb and 200--350 ppb, respectively. Also H2O2 and O3 concentrations decreased by about 35% and 42% respectively from the levels of 1975 to 1987. Model simulations indicated that the reduction of NMHC and CO emissions from industrial and automotive sources have resulted in the decrease in H2O2 levels during the late 1970's--1980's, whereas the reduction of NMHC, CO and NOx emissions have resulted in lower O3 levels. The role of H2O2 as an oxidant for SO2 and as a possible toxin on trees in Los Angeles and surrounding mountains during the past decades are discussed. From the historic data of atmospheric levels of sulfate in Los Angeles, we suggest that the decrease in the levels of H2O2 and O3 may have contributed to the decrease in sulfate levels during the 1980's. ¿ American Geophysical Union 1990