The step-scan time-resolved FTIR emission spectroscopy is used to characterize systematically the H2CO channel for the reactions of O(P-3) with various alkenes. IR emission bands due to the products of CO, CO2, and H2CO have been observed in the spectra. H2CO is identified to be the primary reaction product whereas CO and CO2 are secondary reaction products of O(P-3) with alkenes. A general trend is observed in which the fraction yield of the H2CO product increases substantially as the reactant alkene varies from C2H4, C3H6, 1-C4H8, iso-C4H8, to 1-C5H10. The formation mechanism of the H2CO is therefore elucidated to arise from a 3,2 H-atom shift followed by breaking of the C-1-C-2 bond in the initially formed energized diradical RCH2-CHCH2O*. The 3,2 H-atom shift may become the dominant process with the more rapid delocalization of the energy when the hydrocarbon chain of the alkene molecule is lengthened.