Conformational energies, structural parameters and vibrational frequencies for the tgg’, ggg’, ttg, and ttt conformers of CH(3)XCH(2)CH(2)YH (X, Y = O, S) have been determined by MO ab initio calculations at the MP2/6-31G" level. The results show that the relative strength of the intramolecular X ... Y interaction in tgg’ and ggg’ conformers diminishes gradually along the series of molecules (O,O) --> (O,S) --> (S,O) --> (S,S), as the X ... Y contact distance increases by ca. 2 x 10 pm for each of these substitutions. For the (O,O) and (O,S) molecules, the CX-CC trans form is favored over the gauche arrangement, whereas for the (S,O) and (S,S) molecules, the gauche conformation is more stable. In addition, the oxygen-by-sulfur substitution in the YH bond increases the relative stability of the CC-YH gauche form with respect to the trans arrangement. In turn, gauche,gauche’ arrangements for the XC-CY and CC-YH axes, respectively, lead to closing of intramolecular rings through X ... Y attractive interactions, hence contributing to improve the stability of the conformations with those dihedral arrangements.