The standard molar enthalpy of formation of gaseous 1-adamantyl chloride (1Cl,g) was determined experimentally by means of thermochemical techniques. Ab initio quantum-chemical calculations at the G2-(MP2) level were performed on adamantyl cation (1 + ) and on 1Cl. Combination of these data with other experimental and computational results for adamantane (1H), isobutane (2H), tert-butyl cation (2 + ), and tertbutyl chloride (2Cl) provided purely experimental and purely computational, independent values for the standard enthalpy and Gibbs energy changes for reactions 1 and 2: where X ) Cl. This information was used to determine a value of 162.0 ( 2.0 kcal mol -1 for the standard molar enthalpy of formation of gaseous 1 + , ∆ f H°m(1 + ,g).Holmes and co-workers 1 have recently used appearance energies to determine the standard enthalpy of formation of 1-adamantyl cation (1 + ) in the gas phase, ∆ f H°m(1 + ,g). The value they reported, 152 ( 3 kcal mol -1 , significantly differs from the values obtained by other methods (in the range 158-164 kcal mol -1 ). 1 Inasmuch as carbocations are important chemical species, 2,3 this discrepancy has prompted us to reexamine the problem by means of experimental and computational techniques.Equilibrium techniques, notably ICR (ion cyclotron resonance spectroscopy 4 ), FT ICR (Fourier transform ion cyclotron