Equations constituting the nonequilibrium rate-based separation process problem involving binary and ternary mixtures were analyzed. Multiplicity occurring in the nonequilibrium stage resulted front multiplicity faking place in the equilibrium calculations at the interface. This is achieved by demonstrating a one-to-one correspondence between the bulk and interface compositions in a nonequilibrium stage. The analysis of the film model for the mass-transfer problem where the fluxes are obtained using the Maxwell-Stefan equations showed solution uniqueness for this equation sec. It also showed that the imposition of the mass-transfer equations does nothing to cause additional multiplicities.