The heats of formation of the 1- and 2-adamantyl cations and of the l-adamantyl radical have been measured directly by electron impact appearance energies. The values are Delta(f)H(o)[1-adamantyl(+)] = 152 +/- 3 kcal mol(-1), Delta(f)H(o)[2-adamantyl(+)] = 171 +/- 3 kcal mol(-1) and Delta(f)H(o)[1-adamantyl＇] = 12 +/- 3 kcal mol(-1). The results also lead to a value of 16 kcal mol(-1) for the heat of formation of the 2-adamantyl radical. Collision-induced dissociation mass spectra of the 1- and 2-adamantyl cations allow low internal energy ions to be distinguished. However, it is shown that at higher internal energies the two ions share a common potential energy surface. This is supported by the statistical mixing of all hydrogen and deuterium atoms, prior to the lowest energy dissociation channel, in the 1-adamantyl-3d(1)(+) isotopomer.