The transport model for reverse-osmosis membranes and nanofiltration membranes was reformulated to include the large changes in the chemical activity of solutes in solutions employed to test these membranes. The approach presented herein uses a solution-diffusion model for membrane transport and a more accurate representation of the test solution and the partitioning of the solutes into the membrane. This approach corrects the simplifying assumption in the Merten model of highly diluted solutions and predicts the large changes in the passage of solutes due to increased solute concentration. These changes are due to the large changes in the activity coefficients of the solutes outside the very dilute regions. These changes can significantly influence salt and water-transport coefficients (e.g., A- and B-values).