State-of-the-art electronic structure methods have been applied to obtain the first high-quality theoretical results for substituent effects in pi-stacking interactions. The sandwich configurations of benzene dimer, benzene-phenol, benzene-toluene, benzene-fluorobenzene, and benzene-benzonitrile have been studied using correlation consistent basis sets augmented by multiple diffuse functions, namely aug-cc-pVDZ and aug-cc-pVTZ, at the second-order perturbation theory (MP2) level. Coupled-cluster computations with perturbative triples [CCSD(T)] were performed and combined with the above MP2 calculations to estimate the CCSD(T)/aug-cc-pVTZ binding energies, which should be accurate within several tenths of one kcal/mol. All substituted dimers bind more strongly than benzene dimer, with benzene-benzonitrile binding the most strongly. Both electrostatic and dispersion interactions contribute to the increased binding of the monosubstituted dimers.