HF laser emission was observed in the reaction of C2H3F with O(3P) produced by the flash photolysis of NO2 (λ >300 nm). Several P(J) lines were present in the 1 → 0 band. The laser intensity was found to increase linearly with flash energy. Analyses were made of the stable products of the reaction in both the flash photolysis system and in a discharge-flow system where O(3P) was produced by the N + NO reaction. The observed HF laser emission is believed to result from the following reactions: NO2 + hv → NO + O(3P); O(3P) + C2H3F → C2H3FO† → HF† + CH2CO, ΔH = -106 kcal/mol, where C2H3FO† may be vibrationally excited fluoroethylene oxide, CH2FCHO, or CH3CFO. On the basis of appearance time measurements for the laser lines in a grating-tuned cavity, the HF population ratio between v = 1 and v = 0 was determined to be N1/N0 = 0.52 ± 0.02, which is significantly higher than the value expected statistically. This is interpreted as resulting from the occurrence of a direct three-centered HF elimination from the initial adduct, OCHFCH2, taking place concertedly with the attack by the O atom. The mechanisms of the reaction of O atoms with other fluorinated ethenes are also discussed.