The formation of H2O2 and organic peroxides in the reaction of O3 with trans-2-butene and naturally occurring alkenes has been studied using a 31 m3 reaction chamber. H2O2 and organic peroxides were found to be products of the O3 reaction with trans-2-butene, isoprene, &agr; and &bgr;-pinene, and limonene. Water is necessary for the formation of H2O2 and most of the H2O2 is formed via a route that does not involve HO2 radicals. Our results indicate that the reaction of O3 with natural alkenes may be a significant source of atmospheric H2O2, particularly in forest and rural areas.