A method for measuring the heats of formation of triradicals using energy-resolved collision-induced dissociation (CID) of chloro-substituted biradical negative ions is described. This method is applied to the determination of the heat of formation of 1,3,5-trimethylenebenzene, which was generated by CID of the 5-chloromethyl-m-xylylene ion. The measured CID threshold energy for chloride loss (0.83 +/-0.07 eV) is combined with the electron affinity of the 5-chloromethyl-m-xylylene biradical (1.120 +/-0.059 eV) to give a heat of formation of the triradical of 111.0 +/-4.1 kcal/mol that agrees with the bond additivity value of 109.3 +/-2.1 kcal/mol. The measured heat of formation indicates a third C-H bond dissociation energy (BDE) in 1,3,5-trimethylbenzene of 88.2 +/-5.0 kcal/mol, indistinguishable from the C-H BDE in toluene or the first or second C-H BDEs in m-xylene. The results are in agreement with the predictions made on the basis of simple qualitative and high-level molecular orbital theories that predict negligible interaction between the unpaired electrons in the high-spin triradical.