The enthalpy of formation of m-quinodimethane (m-xylylene) has been determined using two different gas-phase approaches. The first involves combining the electron affinity of the biradical (0.919 +/- 0.008 eV, Wenthold, P. G.; Kim, J. B.; Lineberger, W. C. J. Am. Chem. Sec. 1997, 119, 1354) with the acidity of the 3-methylbenzyl radical. The acidity of the 5-methylbenzyl radical was determined by bracketing the proton affinity of m-xylylene ion, prepared from the reaction of m-xylene and atomic oxygen ions in a flowing afterglow triple quadrupole apparatus. Deuterium labeling and reactivity studies show that 75% of the H-2(+) abstraction product formed in the reaction is the m-xylylene ion, with the rest being 3-methyl-alpha ,n-dehydrotoluene ions resulting from [alpha, ring] hydrogen abstraction. The m-xylylene negative ion underwent reactions similar to those observed for 3-methylbenzyl anion and for other open-shell ions. The m-xylylene ion also reacted with mesitylene by H-2(+) transfer. The acidity of the 3-methylbenzyl radical was 382.5 +/- 2.1 kcal/mol, which leads to an enthalpy of formation of m-xylylene of 80.1 +/- 3.8 kcal/mol. The enthalpy of formation of m-xylylene was also determined to be 81.2 +/- 3.0 kcal/mol using collision-induced dissociation (CID) threshold energy measurements with the 3-(chloromethyl)benzyl ion. The measured enthalpies of formation indicate a second C-H bond dissociation energy in m-xylene of 90.7 +/- 2.9 kcal/mol, only slightly higher than the first (90.1 +/- 1.7 kcaymol). The fact that the strength of the second C-H bond in the 3-methylbenzyl radical is essentially the same as that in m-xylene indicates that the interaction between the two unpaired electrons is negligible, as would be expected for a triplet biradical.