The reaction of chlorine radicals with acetylene in air in the absence of oxides of nitrogen result in the formation of ozone. No ozone is observed when chlorine radicals react with methylacetylene or ethylacetylene under similar conditions. Formyl chloride is observed in all systems. A mechanism is proposed for the major products observed in the acetylene reaction. The addition of chlorine to acetylene followed by molecular oxygen addition results in the formation of the peroxychloroethylene radical. A possible explanation for O3 production is elimination of an oxygen atom from such radicals with excess energy, followed by reaction with molecular oxygen. In the case of substituted acetylenes the larger radicals formed may be able to dissipate the excess energy. Without the formation of atomic oxygen, ozone is not formed. The study demonstrates another mechanism through which ozone may be formed in addition to NO2 photolysis. More work is required to more fully clarify the mechanism for O3 production. (Copyright (c) 1989 Air & Waste Management Association.)