This work discusses a new approach to model the rejection of uncharged solutes by nanofiltration membranes. A mathematical description of the solute is developed - Geometric radius - in which the molecular geometry is represented by a prolate revolution ellipsoid and a possible preferential orientation (orientation angle) of the molecule during permeation through the membrane is considered in the calculations. The model simulations suggest that the target solute rejection depends strongly on its preferential orientation, especially for elongated molecules due to the higher difference in their Geometric radii. In order to evaluate the model, the rejection of defined solutes (n-alcohols and di-alcohols) with different chain length, polarity, symmetry and functional group position was studied experimentally. It was found that the common approach to model the solute rejection, using the molecular Stokes radius, failed to predict the experimental results for the n-alcohols studied, but was applicable for some of the di-alcohols. On the other hand, the Geometric model proposed allowed in all cases for a better description of the experimental rejection data due to its extra degree of freedom - the molecular orientation angle during membrane permeation. Based on these calculated orientation angles, which were dependent of the transmembrane pressure applied, the model could be used as an interpretative too], allowing for a better understanding of the dominant factors that determine the rejection of uncharged solutes with different molecular geometry. (C) 2005 Elsevier B.V. All rights reserved.