The exact scheme to calculate the product composition and the varying overall retention coefficient by the pressure driven membrane separation of the solutions containing one and two solutes is proposed. The scheme avoids many of the inconveniences for choosing optimal separating conditions for all the pressure driven methods, especially in the dead-end mode of operation. The results of these calculations concerning the permeate and retentate composition as well as the change in the overall retention coefficient (of the solutions containing two solutes) have been analyzed and discussed.