Ab initio calculations are carried out using a doubly polarized basis set. Dispersion, evaluated by second‐order Møller–Plesset perturbation theory (MP2), is found to have a profound influence on the stabilities and structures of the H‐bonded complexes. The contribution of dispersion to the H‐bond energies of H2S‐‐HF and H2S‐‐HCl is 44% and 69%, respectively, placing this attractive term second in magnitude only to electrostatics. Reductions of the intermolecular distance of 0.17 and 0.34 Å result from inclusion of correlation effects. Nevertheless, the influence of dispersion upon the angular characteristics of the complexes is rather minor as the relative orientations of the subunits are controlled chiefly by electrostatic factors. The HF‐‐HSH geometry appears to be a true minimum on the potential energy surface but is much less stable than the H2S‐‐HF structure. Comparison of the above systems with previous results for H2O‐‐HF and H2O‐‐HCl reveals a number of regular patterns. Replacement of either first‐row atom of H2O‐‐HF with one from the second row equally diminishes the strength of the H bond; a further reduction to roughly half of the ΔE for H2O‐‐HF occurs when both O and F are exchanged. Comparison between the calculated and observed X‐‐Y distances suggests that the relative changes due to substitutions of O and F by S and Cl are predicted very well by MP2, indicating that this approach is capable of accurately reproducing relative (if not absolute) values of R(X‐‐Y) as well as ΔE. The contribution of dispersion to the interaction energy is magnified by each substitution by a second‐row atom; these exchanges also produce drastic increases in the correlation‐induced contraction of the H bond.