We obtain an expression for the chemical potential which shows the effect that a molecule, when it is introduced into a solution, produces over the entire system. It has been discussed whether a term kT(r(beta) - r(alpha)), which accounts for the differences among the molar volumes of the components, should be used-to calculate the transfer free energies between two phases alpha and beta. We show that both quantities,with and without kT(r(beta) - r(alpha)), give relevant information about the solution process.