Abstract The 298 K heats of formation of the carbynes CH, CF, and CCl have been determined from measurements of the chloride dissociation energies of the halocarbene anions, CHCl ·−, CFCl ·−, and CCl 2 ·−. The chloride dissociation enthalpies were measured by energy-resolved collision-induced dissociation in a flowing afterglow—triple quadrupole instrument to be 37.7 ± 2.3, 21.9 ± 1.1, and 33.4 ± 2.3 kcal/mol, for CHCl ·−, CFCl ·−, and CCl 2 ·−, respectively. The dissociation enthalpies were combined with the known heats of formation and electron affinities of the corresponding halocarbenes and other thermochemical data to give: Δ H f,298 (CH) = 142.2 ± 3.2, Δ H f,298 (CF) = 60.6 ± 3.4, and Δ H f,298 (CCl) = 105.9 ± 3.1 kcal/mol. The experimental results for all three carbynes are in good agreement with the predicted heats of formation obtained from G2 calculations. The measured values for CH and CF are in excellent agreement with other experimental values, whereas results for CCl do not support the heats of formation for this carbyne given in current thermochemical data compilations. The halogen substituent effects on carbyne stabilities and on the sequential C–H bond energies of CH 3F and CH 3Cl are shown to be dominated by π-donor interactions.