The stable products of the low-pressure (4-8 torr (1 torr = 133.33 Pa)) gas-phase reactions of ozone with ethene, propene, 2-methylpropene, cis-2-butene, trans-2-butene, trans-2-pentene, 2,3-dimethyl-2-butene, and 2-ethyl-1 -butene have been identified by using a photoionization mass spectrometer coupled to a stirred-flow reactor. The products observed are characteristic of (i) a primary Criegee split to an oxoalkane (aldehyde or ketone) and a Criegee intermediate,of the Criegee intermediates such as unimolecular decomposition, secondary ozonide formation, etc., and (iii) secondary alkene chemistry involving OH and other free-radical products formed by the unimolecular decomposition of the Criegee intermediates. The secondary OHalkene412 reactions account for a significant fraction of the alkene ( C a b ) consumed and lead to characteristic products such as C, dioxoalkanes [R'C(O)C(O)R'', at m/e (C,Hb + 30)], C, acyloins [R'C(O)CH(OH)R", at m/e (C,H2, + 32)], and C, alkanediols [R'CH(OH)CH(OH)R", at m/e (C,H2, + 34)]. C, oxoalkanes and C, epoxyalkanes observed at m/e (C,H2, + 16) are probably formed primarily via epoxidation of the alkene by 03. A general mechanism has been proposed to account for the observations.There is considerable evidencel-10 that the gas-phase reactions of ozone with alkenes can be described, at least in part, by the Criegee mechanism shown in eq 1 and 2 for solution-phase ozo-