The CH/π interaction energies between benzene and halomethanes (CH 2Cl2 and CHCl3) were accurately determined. Two-color ionization spectroscopy was applied to the benzene-CH 2Cl2 and -CHCl3 clusters, and the binding energies in the neutral ground state, i.e. the CH/π interaction energies in these model cluster systems, were precisely evaluated on the basis of the dissociation threshold measurements of the clusters in the cationic state and the ionization potential value of the bare molecule. The experimentally determined interaction energies were 3.8 ± 0.2 and 5.2 ± 0.2 kcal mol-1 for benzene-CH2Cl2 and -CHCl3 respectively, and the remarkable enhancement of the CH/π interaction energy with chlorine-substitution was quantitatively confirmed. The experimental interaction energies were well reproduced by the high-level ab initio calculations. The theoretical calculations clarified the unique nature of the activation of the CH/π interaction by the chlorine-substitution.