The intrinsic factors governing the diastereofacial selectivity of 2-methyl-5-X-2-adamantyl cations (X = F (I-F), Si(CH3)(3) (I-Si)) toward a representative nucleophile, i.e., methanol, have been investigated in the gas phase at 750 Torr and in the 20-80 degreesC temperature range. The kinetic results indicate that CH3OH addition to I-F proceeds through tight transition structures (TSFsyn and TSFanti) characterized by advanced C-O bonding. The same interactions are much less pronounced in the comparatively loose transition structures involved in the CH3OH addition to I-Si (TSSisyn and TSSianti). The experimental evidence indicates that the activation barriers for the anti addition to I-F and I-Si are invariably lower than those for the syn attack. Large adverse entropic factors account for the preferred syn diastereoselectivity observed in the reaction with I-F. Entropy plays a minor role in the much looser transition structures involved in the reaction with I-Si, which instead exhibits a preferred anti diastereoselectivity. Comparison of the above gas-phase results with related theoretical and solution data suggests that the diastereofacial selectivity of I-F and I-Si measured in solution arises in part from the differential solvation of the two faces of the pyramidalized ions.