The product branching fractions for the reaction of atomic oxygen with ethene, O((3)P) + C(2)H(4)--> CH(3) + HCO (1a), --> H + CH(2)CHO (1b), --> H(2) + CH(2)CO (1c), have been investigated at room temperature (295 +/- 4 K) and pressures from 1 to 4 Torr (with N(2) or He buffer) by a laser photolysis-photoionization mass spectrometry method. From the yield of CH(3) radical, phi(CH(3)), the branching fraction for (1a) was determined to be 0.53 +/- 0.04 and no apparent pressure dependence was found from 1.5 to 4.0 Torr (N(2) buffer). The ratio of the HCO yield to that of CH(3), phi(HCO)/phi(CH(3)), was measured to be less than unity and increased as pressure increased (approximately 0.7 at 1 Torr and approximately 0.9 at 4 Torr [He]) suggesting prompt dissociation of the hot HCO radical (to H + CO) formed by channel (1a) at low pressures. An interpretation which reduces the large discrepancy among branching fractions reported for low pressure region is presented. The existence of the molecular H(2)-elimination channel (1c) was confirmed. The branching fraction for channel (1c) was determined to be 0.019 +/- 0.001 by the yield of CH(2)CO and was independent of pressure from 1.0 to 4.0 Torr (He buffer). As a side result, the yield of CH(3) radical from O((1)D) + C(2)H(4) reaction was also determined.