In this paper, a model is developed for the retention of organic molecules with a given nanofiltration membrane at different pressures as a function of the molecular weight. The Spiegler-Kedem transport equations were used to derive the reflection coefficient, the maximal retention that would theoretically be obtained at infinite transmembrane pressure, from experimental retention values for a large set of molecules with the effective diameter of the molecule as a size parameter. Secondly, the pore size distribution of the membrane is derived from the experimental reflection coefficients. This allows to calculate the reflection coefficient for a molecule with a given effective diameter. Since this parameter is not readily available, a correlation between the effective diameter and the molecular weight has been established and introduced in the model equations.