The relative hydrogen-bond (HB) properties of sulfoxides and selenoxides have been investigated experimentally using data retrieved from the Cambridge Structural database and theoretically through density functional calculations at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-311++G(3df,3pd) level. The HB are significantly shorter (stronger) in selenoxides (d(O...H) = 1.78 (3) (A) over circle) than in sulfoxides (d(O...H) = 1.85 (2) (A) over circle). The HB directionalities and linearities observed in the solid state for the two functionalities are very similar. The spatial and molecular surface minima of the electrostatic potential are, respectively, 43.9 and 23.4 kJ/mol more negative in dimethyl selenoxide (DMSeO) in comparison with that in dimethyl sulfoxide (DMSO). The investigation of the S(Se)O bond's electronic structure using the Natural Bond Orbital (NBO) approach shows that negative hyperconjugation of the type no --> delta*S(Se)-C is much more important in DMSO than that in DMSeO. In the HB complexes, the NBO analysis shows competition between n(O) --> delta*S(Se)-C delocalizations associated to hyperconjugation and n(O) --> delta*HF delocalizations related to hydrogen-bonding. The NBO energetic analysis of the HB complexes demonstrates that the H...O interaction is significantly greater in DMSeO compared to that in DMSO. The computed thermodynamic parameters of HB; complexation support the better HB ability of selenoxides since the deltaDeltaH(298)degrees and deltaDeltaG(298)degrees are, for the three HB donors used (HF, H2O, and 29 p-fluorophenol) always significantly in favor of DMSeO. The theoretical deltaDeltaH(298)degrees, and deltaDeltaG(298)degrees, calculated for 29 DMS(Se)O HB interactions with p-fluorophenol, respectively, of 8.5 and 6.0 kJ/mol, compares reasonably well with the corresponding experimental data in solution of 2.8 and 5.0 kJ/mol. The theoretical deltaDeltav(XH) clearly confirm the various observations, the deltaDeltav(OH) value of 111cm(-1) I calculated from the HB; complexes with H2O being close to the experimental deltaDeltav(OH) of about 110(20)cm(-1).