We used ab initio and density functional theory (DFT) methods to perform calculations on dimethyl ether and dimethyl sulfide dimers and trimers. A total of five minima were identified for each dimer that exhibited two different types of hydrogen bonds depending on whether the hydrogen atom involved was located in the molecular plane. The most stable dimer for dimethyl ether was found to be a structure where two molecules lie in two perpendicular planes and possess an interaction energy of -12.0 kJ/mol. On the other hand, the most stable structure in the dimethyl sulfide dimer exhibits two molecules in an antiparallel arrangement and an interaction energy of -17.2 kJ/mol. On the basis of the calculations, the ether tends to interact with the hydrogen atoms in the molecular plane whereas the sulfide interacts preferentially with hydrogen atoms outside it. The interaction with the atoms in the plane is similarly strong in both molecules; however, the sulfur atom interacts more strongly with the hydrogen atoms located outside the molecular plane; this results in the dimer of dimethyl sulfide being more stable than that of dimethyl ether. We examined three minima for the corresponding trimers, where pairwise nonadditive contributions were found to be negligible and mostly of the repulsive type. Also, no appreciable cooperative effects were observed. The most stable trimer for the ether was found to have an interaction energy of -24.8 kJ/mol and that for the sulfide one of -34.4 kJ/mol; consequently, the CH...S interaction is stronger than the CH...O interaction, both in the dimer and in the trimer.