The reaction of atomic oxygen O(³P) with ethene, propene and (E)-but-2-ene has been studied between 10 and 50 mbar total pressure using the alkene as both reactant and bath-gas. NO₂ was used as the source of atomic oxygen, and the decay of NO₂, which is sensitive to the radical products from the reaction of O atoms with alkenes, was followed by laser-induced fluorescene. Branching ratios for total radical production were obtained for the propene and (E)-but-2-ene systems. The radical branching ratio for the propene system was observed to decrease with pressure by a factor of about two over the range measured, falling from 0.64 at 10 mbar to 0.36 at 40 mbar total pressure. The radical branching ratio for the (E)-but-2-ene system also decreased by a factor of two, falling from a value of 0.22 at 10 mbar to 0.11 at 30 mbar. The results for the ethene system were inconclusive, in that only a range for the branching ratios for production of CH₃+ HCO and H + CH₂CHO could be obtained. These findings were confirmed by extensive modelling of the results.