An ab initio study of the conformational behavior of alpha- and beta-anomeric linkages in C-, N-, and S-glycosyl compounds has been carried out on axially and equatorially 2-substituted derivatives (2-ethyl, 2-methylamino, 2-thiomethyl, and 2-methylammonio) of tetrahydropyran as models. The geometry of the conformers about the anomeric C-X bond was determined by gradient optimization at the SCF level using the 6-31G* basis set. The potential of rotation has been calculated using the 6-31G* and 6-31+G* basis sets. Vibrational frequencies were calculated at the 6-31G* level and used to evaluate zero-point energies, thermal energies, and entropies for minima. Variations in calculated valence geometries for the compounds, display structural changes distinctive for the anomeric and exo-anomeric effects. Differences between bond lengths and bond angles for different conformers correlate with the importance of the lone pair delocalization interactions. The calculated conformational equilibria have been used to estimate the magnitudes of the anomeric, reverse anomeric, and exo-anomeric effects. It was found that the anomeric effect decreases in the following order : chlorine > methoxy similar to fluorine > thiomethyl > methylamino > ethyl > methylammonio, with the methylamino, ethyl, and methylammonio groups exhibiting reverse anomeric effects. The sc preference of the methyl group over the ap orientation around the Cl-C bond in 2-ethyltetrahydropyran is assumed to be entirely on basis of steric interactions. The exo-anomeric effect is expected to be present when the preference for the sc conformation is larger than that for the ethyl group. Thus, the exo-anomeric effect decreases in the order methoxy similar to methylamino > thiomethyl. The methylammonio group does not show an exo-anomeric effect.