The generation of conjugate olefins from the reactions of propyl (reaction 1) chloroethyl (reaction 2) radicals with O-2 has been investigated as a function of total pressure (0.4-700 Torr) at 297 +/- 2 K. The experiments were carried out by UV irradiation of mixtures of propane (or ethyl chloride), Cl-2, and O-2 to generate alkyl radicals. Propylene from reaction 1 was measured by FTIR spectroscopy, while vinyl chloride from reaction 2 was monitored by both FTIR and gas chromatographic analysis. At pressures where the formation of propylperoxy radicals is near the high-pressure limit, the propylene yield from reaction 1 was inversely dependent on total pressure (Y-C3H6 proportional to P--0.68+/-0.03), providing that it is formed via rearrangement of an excited propylperoxy adduct that can also be stabilized by collision. The vinyl chloride yield decreased from 0.3 +/- 0.1% at 1 Torr to <0.1% at 10 Torr. Because the formation of chloroethylperoxy radicals is in the fall-off region over this pressure range, the vinyl chloride yield cannot be ascribed unambiguously to an addition-elimination process. The propylene yield from reaction 1 is 2-4 times smaller than the ethylene yield from C2H5 + O-2 over the pressure range 0.4-100 Torr, while the vinyl chloride yield from reaction 2 is 40 times smaller between 1 and 10 Torr. This is consistent with more efficient stabilization of the excited propylperoxy relative to the ethylperoxy adduct caused by the presence of additional vibrational modes, The markedly smaller ambient temperature vinyl chloride yield from reaction 2 may result from a combination of more efficient stabilization resulting from the lower frequency of the C-Cl bond and reduction of the C-H bond reactivity upon Cl substitution.