The total process time for the concentration of a macrosolute solution by ultrafiltration and the removal of a microsolute by diafiltration, is calculated given that the limiting flux, J(lim), is related to the macrosolute bulk concentration, C, by J(lim) = a In (C-lim - C(1 - sigma)/C sigma) where a is a constant, C-lim is an apparent limiting 'wall' concentration of the macrosolute and sigma is the macrosolute rejection coefficient. It is shown that the optimum macrosolute concentration for commencing the diafiltration increases as the rejection coefficient decreases. Previous attempts to model the effect of rejection coefficient on the optimum concentration are shown to be derived in an inconsistent manner and under-predict the optimum concentration. It is also shown that the optimum macrosolute concentration increases as more microsolute is removed in the diafiltration step. (C) 1999 Elsevier Science B.V. All rights reserved.