The CH/π interaction energies between benzene and halomethanes (CH₂Cl₂ and CHCl₃) were accurately determined. Two-color ionization spectroscopy was applied to the benzene–CH₂Cl₂ 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.