A Monte Carlo simulation of statistical perturbation theory (SPT) has been applied to iuvestigate solvent effects on the relative free energies of solvation of La3+ to Nd3+ ion mutation in several solvents. Comparing the relative free energies for interconversion of La3+ to Nd3+, in H2O (TIP3P) from this Letter with computer simulation and experiments, there is good agreement among the three studies, There is also reasonable agreement between calculated structural properties from this Letter and other works. For these ion pairs, the Born's function of the solvents and the differences in solvation dominate the differences in the relative free energies of solvation and partition coefficients.