Absolute heats of formation for alpha,2-, alpha,3-, and alpha,4-dehydrotoluene biradicals have been determined from the measured threshold energies for dissociation of chloride, bromide, and iodide ion from the corresponding o-, m-, and p-halobenzyl anions in the gas phase. The apparent heats of formation derived for the alpha,2- and alpha,4-dehydrotoluene biradicals exhibit a dependence upon the particular halide ion used for the threshold energy measurement (decreasing with increasing halide atomic number), while the final heat of formation obtained for the alpha,3-dehydrotoluene biradical is invariant with changes in the halide. The 298 K heats of formation derived from the iodobenzyl anion results for alpha,2-, alpha,3-, and alpha,4-dehydrotoluene are all found to be 103 +/- 3 kcal/mol. This value is in fair agreement with the predicted heats of formation for the ground state of each biradical obtained from MCSCF calculations (105-106 kcal/mol) and significantly lower than the value of 107.6 +/- 1.7 kcal/mol predicted by a simple bond energy additivity calculation. The MCSCF calculations indicate alpha,2- and alpha,4-dehydrotoluene to be ground-state triplet biradicals with open-shell singlets lying 7.4 and 8.1 kcal/mol higher in energy, respectively, while alpha,3-dehydrotoluene is found to be a ground-state singlet with the triplet lying 3.0 kcal/mol higher in energy. The halide ion dependence of the apparent heats of formation for the alpha,2- and alpha,4-dehydrotoluene biradicals is attributed to the spin-forbidden nature of the dissociation reactions that produce them. The intersystem crossing required to form ground-state triplet products from the halobenzyl anion precursors is associated with a reverse activation energy and/or a kinetic shift in the reaction onset due to slow unimolecular decomposition kinetics. Both effects would be expected to diminish with the heavier halides. In contrast, dissociation of a m-halobenzyl anion to produce alpha,3-dehydrotoluene is spin-allowed, so the reaction occurs at the true thermochemical limit.