The absolute heat of formation at 298 K for ground-state triplet cyanocarbene, HCCN, has been determined from a measurement of the chloride dissociation energy of ClCHCN-. Analysis of the energy-resolved collision-induced dissociation cross section as a function of center-of-mass collision energy in a flowing afterglow triple quadrupole instrument gives a chloride dissociation enthalpy of 43.7 +/- 2.5 kcal/mol. Proton-transfer bracketing experiments were used to determine a gas-phase acidity, DeltaH(acid), of 357.7 +/- 2.0 kcal/mol for ClCH2CN. The heat of formation at 298 K for ClCH2CN was determined from collision-induced dissociation of a series of protonated nitriles to be 25.5 +/- 3.8 kcal/mol. The chloride ion dissociation enthalpy and the heat of formation and gas-phase acidity of ClCH2CN are combined in a simple thermochemical cycle to give an absolute heat of formation for HCCN of 115.6 +/- 5.0 kcal/mol. High level theoretical calculations were performed in support of the experimental study at the G2 (I), CBS-Q (II), CBS-APNO (III), B3LYP/6-31G* (IV), and B3LYP/6-311++G** (V) levels of theory. The compound methods, I-III, give predictions for the acidity and heat of formation of ClCH2CN and for the heat of formation of the triplet ground state and first excited singlet state of HCCN that are in good agreement with experiment. The density functional theory predictions (IV,V) for these quantities are fair at best. The heat of formation of (HCCN)-H-3 is used to derive additional thermodynamic quantities including a C-H bond dissociation enthalpy in CH2CN of 107.3 +/- 5.4 kcal/mol and a singlet-triplet splitting for HCCN of 11.1 +/- 5.8 kcal/mol.