The production of CO from the reaction of O(3P) atoms with propargyl chloride and bromide has been measured and compared with that formed in the analogous reaction involving methylacetylene. A cw CO laser was used to probe the absolute concentration and vibrational energy content of the CO. These results show that CO molecules formed in these three reactions contain essentially the same amount of vibrational energy, having a Boltzmann temperature of about 2400 ± 100 K. The rates of CO production in these reactions, however, differ considerably and have the following relative rates: methylacetylene: propargyl chloride: propargyl bromide = 1 : 0.25 : 0.12. These data suggest the possible simultaneous occurrence of other parallel channels, presumably, HX-elimination from the excited ketene intermediates via the following four-centered mechanism, viz., (Figure presented.) where X = Cl, Br. The elimination product, C3H2O, is believed to be too stable to generate a CO molecule subsequently and thus reduces the rate of CO formation in these two cases.