An original method has been proposed to determine the dynamic permeability of membranes. Experiments were run under different operating conditions (various transmembrane pressures, membranes, concentrations, and solutes), and the experimental data were processed using this dynamic permeability model. The results show that permeability defined in this manner reflects the differences in the membrane behaviour from pure water to a solution or from one solution to another. With dynamic permeability data, membrane condition can also be evaluated after use without the need to run experiments with pure water.