The formation of H2O2and organic peroxides in the reaction of O3with trans‐2‐butene and naturally occurring alkenes has been studied using a 31 m3reaction chamber. H2O2and organic peroxides were found to be products of the O3reaction with trans‐2‐butene, isoprene, α and ß‐pinene, and limonene. Water is necessary for the formation of H2O2and most of the H2O2is formed via a route that does not involve HO2radicals. Our results indicate that the reaction of O3with natural alkenes may be a significant source of atmospheric H2O2, particularly in forest and