The HCO product of the reaction of O(P-3) with ethene has been detected by cavity ring-down spectroscopy using its A-X transition. For propene a somewhat smaller yield of HCO was obtained but the overall rate constant is much larger. The yield of HCO in this reaction is quite small (similar to 0.05). Moreover, a large number of other alkenes were tried with negative results. The failure of the 1,2 H atom shift followed by breaking the 1,2 bond implies that the unimolecular decomposition has found a more favorable channel. The proposed mechanism is as follows. For an alkene of the form RCH2CH=CH2 the first step is attachment of the O(P-3) to the terminal carbon atom, C-1. Then, intersystem crossing occurs and finally a H atom shifts from C-3 to C-2 and not from C-1 to C-2. In this way a molecule of formaldehyde and an alkene shorter by one carbon atom are formed.