The Sanchez-Lacombe equafion-of-state is known to describe thermodynamic properties of molecular fluids of arbitrary size, mainly polymer-solvent phase behaviour. On the basis of the Helmholtz energy obtained from the partition function of mixtures, chemical potentials are usually derived in order to compute phase equilibrium conditions at various temperatures and pressures. In this work, it is shown that, whatever the mixing rules considered, the chemical potentials derived in this way are thermodynamically inconsistent. The fugacity coefficients derived from the Sanchez-Lacombe equation-of-state are proposed for calculating consistent phase equilibrium conditions.