A novel methodology has been developed which enables optimization of membrane separations. In multicomponent separation processes, sieving coefficients for the individual solutes, defined as the ratio of the filtrate and feed concentrations, tend to reach optimum values under different process conditions. It is not possible to determine a priori the pair of sieving coefficients which will give the best combination of product yield and purification for a given application. A purification factor-yield diagram for such an optimization has been developed which utilizes a family of curves representing two dimensionless numbers plotted on yield versus purification-factor coordinates. Analysis can be performed with knowledge of only three experimental variables : the filtrate flux and the two solute sieving coefficients. Complete optimization of membrane processes can be achieved by combining these variables with membrane area, process time, and retentate-volume constraints. The methodology should be applicable to ultrafiltration, microfiltration, and high-performance tangential flow (selective) filtration processes.