The solute separation in reverse osmosis/nanofiltration (RO/NF) membrane processes is mainly controlled by both the diffusivity in a pore and the steric partition factor, and the latter is defined by the geometrical probability at which a solute can be accessible into a pore. The rejection of alcohols has been estimated semi-empirically by using the steric partition factor derived by the approximation of molecular shape as a rectangular parallelepiped. However, the approach was not suitable for crown ethers, and in this work the shape of crown ethers was approximated as a disk: the disk radius and disk thickness were developed as new shape parameters. The calculated rejections of crown ethers by using the disk-shaped model corresponded well to the observed rejections, where the used pore radius was calculated on the basis of the rejections of alcohols. Boric acid in non-dissociated form is also assumed to be a disk-shaped molecule, but the calculated rejection by the disk-shaped model was smaller than the observed one. The results suggest that boric acid transports through membrane pores with hydrated water molecules. (C) 2016 Elsevier B.V. All rights reserved.