The parameters of the model comprising the polarization layer and the Spiegier-Kedem-Katchal sky model of membrane transport were determined for a series of nanofiltration membranes. Membranes with different NaCl retentions were chosen. The experiments were performed at the same hydrodynamic conditions using 0. 1 M NaCl as feed. Different approaches to the calculation of parameters are presented, including the concentration dependence of P-s and sigma. It was found that only P-s =f(C-s) substantially improved the fitting of the model. A strong dependence of the real retention coefficient, R, on the electrolyte permeability coefficient, P, was described by the empirical relation R = (I + q(0)P(s)(qi))(-1). No regular relationship between R-obs and the reflection coefficient, sigma, was found. For some membranes the inequality sigma > 1 was obtained. A strong relation between the thickness of polarization layer, I'll, and R was found, expressed by the power function l(pol) = d(0)R(di) (r(2)= 0.986). Thus, in the modeling of membrane transport, one should include the polarization layer into the model instead of applying the ready correlations which do not take into account the membrane properties.