The contribution of atomic chlorine to the chemistry of marine regions as well as the Arctic at ground level at polar sunrise is the subject of a number of recent studies. However, identifying the specific chlorine atom precursors has proven difficult. One potential approach is the measurement of definitive products of chlorine atom reactions, for example with biogenic hydrocarbons. We report here product studies of the chlorine atom reaction with isoprene using ppm concentrations at one atmosphere air and 298 K in a NOx-free system using atmospheric pressure ionization-mass spectrometry (API-MS) as well as GC-MS. 1-chloro-3-methyl-3-butene-2-one (CMBO) is identified as a unique product of this reaction, and there is evidence of the formation of three additional isomers of CMBO as well. Methyl vinyl ketone (MVK) is formed in small yields (9¿5%), consistent with earlier studies of this reaction in which an upper yield of 13% was reported. The stable product expected from allylic hydrogen atom abstraction (measured in earlier kinetic studies to be 15% of the total reaction), 2-methylene-3-butenal, is also tentatively identified using API-MS. Assuming that similar chemistry occurs at the ppb-ppt levels found in the atmosphere, identification of CMBO and/or its isomers in field studies could provide strong evidence of chlorine atom chemistry in low NOx environments where there are also sources of isoprene.¿ 1997 American Geophysical Union