Mathematical models for the calculation of retention and selectivity for nanofiltration of aqueous dye-salt solutions are presented and compared with results from experiments done with commercial nanofiltration membranes and sodium chloride and R-salt as test substances [1] . A simple equation predicts the dependence of retention on feed concentration and permeate flux for single salt solutions . Three simple filtration experiments are sufficient to determine the required parameters . hence a useful model for the membrane producer and user is given .
Due to the Donnan effect both concentrations of mixed solutions of R-salt and NaCI in water determine the retention of each component . It was shown that an optimum in selectivity can occur at a certain flux . The influence of temperature on the permeate flux could be well described by a viscous pore flow. The product of permeate flux and selectivity for a solution of R-salt/ NaCl turned out to be independent of temperature . Experiments with mixed solutions of sodium chloride and an industrial dye showed selectivities up to S=1 .3 . 105 because of the extremely high retention of the dye . The models cited above were only valid for these very high retentions (R>99%) if a pressure-dependent reflection coefficient were assumed .