We report herein the results of an experimental and computational study of adamantylideneadamantane (1) and a variety of substituted ethylenic hydrocarbons. The standard enthalpy of formation in the gas phase as well as the gas-phase basicity (GA) of 1 were experimentally determined for the first time, respectively by calorimetric techniques and FT-ICR spectrometry. In parallel, computational studies at the MP2/ 6-311+G(d,p), G3(MP2), and G3 levels were performed on the neutral (1) and protonated (1H+ ). The agreement with experimental results was very good. The structures of 1 and1H+ were subject to treatment by "Atoms in Molecules" in order to assess the characteristics of the closest H£H interactions involving both adamantane moieties. Also, the secondorder perturbation analysis within the Natural Bond Orbital Theory methodology shows four degenerate charge-transfer interactions between the ó C?H bond of one of the adamantyl subunits towards the ó∗ C?H of the other adamantyl subunit. The standard enthalpies of formation of new adamantyl compounds were obtained using our experimental data. The computational study of a variety of ethylenic compounds including cyclohexylidenecyclohexane and several alkyl-substituted ethylenes using isodesmic and homodesmotic reactions was carried out. This study was extended to their proton affinities and gas-phase basicities.