The mechanism of the reactions of O(P-3) with three isomeric butenes (trans-2-, iso- and 1-butene) was investigated observing the yields of CH3 and C2H5 radicals by using a photoionization mass spectrometry at room temperature (298 5 K) and pressure of 5 Torr (He buffer). The branching fractions for the CH3 channel were determined as 0.52 +/- 0.10, 0.24 +/- 0.08, and 0.05 +/- 0.01 for the reactions of O atoms with trans-2-butene, isobutene, and 1-butene, respectively. Also, the yield of the C2H5 for these reactions was estimated as 0.29 +/- 0.05 for both trans-2-butene and 1-butene but was quite low for isobutene (< 0.05). The possible mechanisms are discussed based on the present results as well as the previous studies.