The kinetics of the allyl + HO2 bimolecular reaction, the thermal decomposition of C3H5OOH, and the unimolecular reactions of C3H5O are studied theoretically. High-level ab initio calculations of the C3H5OOH and C3H5O potential energy surfaces are coupled with RRKM master equation methods to compute the temperature- and pressure-dependence of the rate coefficients. Variable reaction coordinate transition state theory is used to characterize the barrierless transition states for the allyl + HO2 and C3H5O + OH reactions. The predicted rate coefficients for allyl + HO2 → C3H5OOH → products are in good agreement with experimental values. The calculations for allyl + HO2 → C3H6 + O2 underpredict the observed rate. The new rate coefficients suggest that the reaction of allyl + HO2 will promote chain-branching significantly more than previous models suggest.