The H, CH2CHO, CO, and OH products of the reaction of O(P-3) atom with alkenes were studied by laser-induced fluorescence (LIF) under single-collision conditions. The average kinetic energies of the H atoms were 10-12 kcal/mol. The CO and OH rotational state populations were characterized by near room temperature Boltzmann distributions. The relative LIF intensities of the various products provide vivid proof of the following mechanism for the reaction of O(P-3) atoms with molecules of the form R'RC=CH2, where R' and R are H or an alkyl group. The O atom attaches itself to the less substituted carbon atom forming a triplet ketocarbene. There is a barrier to the release of an H atom, and the rate of release must compete with the sate of intersystem crossing. If an H atom is not released, following the intersystem crossing an H atom migrates to the adjacent C atom forming an energized aldehyde, R'RCH-CH=O. The aldehyde dissociates unimolecularly forming the pair of radicals R'(.) and the substituted vinery (RCH)-R-.-CH=O or the pair R'RCH and HCO. Some of the latter have enough internal energy to dissociate to H and CO. In a side reaction O(P-3) abstracts H atoms but only from allylic C-H bonds. The most remarkable observation is that chemical reactions that do not involve the side chains such as release of H atoms or breakup of HCO depend sensitively on the length of these chains.