MP2/aug-cc-pVTZ calculations are performed on complexes of YO 3 (Y 5 S, Se) with a series of electron-donating chalcogen bases YHX (X 5 H, Cl, Br, CCH, NC, OH, OCH 3 ). These complexes are formed through the interaction of a positive electrostatic potential region (p-hole) on the YO 3 molecule with the negative region in YHX. Interaction energies of the binary O 3 YÁÁÁYHX complexes are in the range of 24.37 to 212.09 kcal/mol. The quantum theory of atoms in molecules and the natural bond orbital analysis were applied to characterize the nature of interactions. It was found that the formation and stability of these binary complexes are ruled mainly by electro-static effects, although the electron charge transfer from YHX to YO 3 unit also seems to play an important role. In addition, mutual influence between the YÁÁÁN and YÁÁÁY interactions is studied in the ternary HCNÁÁÁO 3 YÁÁÁYHX complexes. The results indicate that the formation of a YÁÁÁN interaction tends to weaken YÁÁÁY bond in the ternary systems. Although the YÁÁÁY interaction is weaker than the YÁÁÁN one, however, both types of interactions seem to compete with each other in the HCNÁÁÁO 3 YÁÁÁYHX complexes.