Retention data for a series of small, neutral solutes in methanol are measured through three different solvent resistant membranes. The neutral solutes include linear alkyl acetates, branched alkyl acetates, and cyclohexyl acetates with molecular weight ranging from 116 to 228 g mol(-1), For those solutes with approximately the same MW but different molecular size, all three membranes show preferential transport of straight-chained solutes over cyclic solutes, while the branched solutes revealed the lowest permeability. In order to correlate the retention with the molecule size or shape of the solutes, six traditional molecular size descriptors (MW, the Stokes diameter, the equivalent molar diameter, the empirical effective diameter, molecular length and radius of gyration) were compared to a new molecular shape descriptor, the "calculated mean size". A strong correlation between molecular shape and solute retention across a range of membranes was only found with the calculated mean size of the various solutes, which was calculated on the basis of three-dimensional molecular projection. In addition, one-parameter and two-parameter regression analyses were performed, which justified the calculated mean size as a suitable model parameter for the modeling of solute retention. (C) 2008 Elsevier B.V. All rights reserved.