The origin of enantioselectivity in the dihydroxylation of H2C=CH(Ph) catalyzed by (DHQD)(2)PYDZ . OsO4 ((DHQD)(2)PYDZ = bis(dihydroquinidine)-3,6-pyridazine) is analyzed theoretically by means of hybrid QM/MM calculations with the IMOMM(Becke3LYP:MM3) method. Twelve different possible reaction paths are defined from the three possible regions of entry of the substrate and its four possible orientations and characterized through their respective transition states. The transition state with the lowest energy leads to the R product, in agreement with experimental results. The decomposition of the interaction energy between catalyst and substrate shows how the selectivity is essentially governed by stacking interactions between aromatic rings, with a leading role for the face-to-face interaction between the substrate and one of the quinoline rings of the catalyst.