The photoionization thresholds measured for the formation of C3H5+ from propylene and cyclopropane are 11.78 and 11.47 eV, respectively. Our best value of the heat of formation of allyl cation is 224.5±1.0 kcal/mole. Lossing's [Can. J. Chem. 49, 357 (1971)] value of 8.07 eV for the ionization energy of allyl radical combined with our results gives 38.4±1.7 kcal/mole for the heat of formation of allyl radical.