The purpose of the work described in this paper is to investigate the utility of ab initio molecular orbital calculations for the prediction of rate constants and activation parameters of reactions occurring in hydrocarbon combustion. The reaction of vinyl radical with oxygen has been chosen because there exist reliable experimental data against which the calculations can be calibrated. The results suggest that good agreement (within a factor of 2) between observed and calculated rate constants can be achieved but only ifa mechanism different from the one previously assumed is employed. The new mechanism involves cyclization of the first-formed vinylperoxy radical to a three-membered-ring dioxiranylmethyl radical rather than the four-membered-ring dioxetanyl radical that was assumed in earlier mechanisms. The agreement of the computed rate constants with existing experimental data, as well as the identification of the new mechanism, would appear to imply that ab initio calculations of the type described can have a useful role in the analysis of combustion processes. Predictions of results expected in shock-tube studies of the reaction are presented. It is shown that shock-tube experiments should provide definitive distinction between the old and new mechanisms for the reaction.