Abstract The effects of selvents on ρ values have been systematically treated as a two-variable problem within a linear free-energy relationship framework. It is assumed that the standard activation free energy is a continuous function of σ, a substituent variable, and ξ, a solvent variable. The applicability of the resultant equations is proved by a test of the dissociation constants (obtained from the literature) of benzoic acids in various solvents. As a result, it is proposed that these effects on ρ values can be explained in terms of the interaction or cross-term of σ and ξ. The application to the reaction rates of a typical Menschutkin reaction, obtained by experiments in ten nonaqueous solvents, suggests that the solvent dependence in these reactivities is much more complicated.