A governing equation describing the mass-transfer fluxes in practical membrane formation is derived using Markoffian and Onsager’s thermodynamic systems. This equation unambiguously demonstrates that isothermal mass-transfer fluxes at the spinodal composition may not be zero for systems operated in the gravitational field. This study suggests that the mass transfer equations derived by Cheng and Gryte are a special case of the present study and are not fully capable of describing the mass transfer in practical membrane formation. Experimental results published in previous literature qualitatively support our assessment. The Limitations of using Markoffian and Onsager’s thermodynamic systems for membrane formation study are also reviewed. In conclusion, new thermodynamic theories fur polymer solutions are needed to quantitatively describe the mass transfer fluxes in practical membrane formation.