H-bonds involving NH3, PH3, OH2, and SH2 as proton acceptor and HF and HCl as donor, as well as the water dimer, are studied by ab initio molecular orbital methods. The influence of dispersion on the properties of these complexes is calculated by second-order Møller —Plesset perturbation theory using a doubly-polarized double-ξ basis set. This theoretical approach yields H-bond lengths in much better agreement with experiment than distances calculated at the SCF level. The contractions in the bond lengths introduced by dispersion grow rapidly as first-row atoms are replaced with second-row analogs. A similar trend is noted in the H-bond energies where the contribution of dispersion ranges between 22% for the smaller systems and 69% for complexes such as H2SHCl. In addition, dispersion is seen to be responsible for a large fraction of the stretch in the HX bond resulting from H-bond formation. On the other hand, the angular characteristics of these complexes are relatively unaffected by dispersion but are controlled, instead, primarily by electrostatic factors.