The influence of pi-pi interactions on the hydrogen bonding ability of pyrimidine and imidazole stacked with substituted benzenes in the T-shaped, parallel-displaced and parallel-sandwich arrangements is studied at the MP2 level of theory. While the dispersion is the major source of the stabilization of the complexes, it appears that the hydrogen bonding ability of the heterocycles is directly related to the electrostatic interaction between the cycles. The dispersion interaction can successfully be modeled by London's formula involving the polarizability. The reactivity descriptor local hardness, introduced in a conceptual DFT context, is found to be an excellent tool for the estimation of the interaction energy. (C) 2004 Elsevier B.V. All rights reserved.