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Abstract: This work systematically investigates and models the retention behavior in organic solvent nanofiltration in solvent mixtures using a dense PDMS-based composite membrane. Experiments with n-alkanes as solutes with different chain lengths (142–339kgkmol−1) were performed. The transport of these solutes is comprehensively quantified not only in pure solvents such as toluene, isopropanol or methanol but also in binary mixtures of isopropanol/toluene and methanol/toluene. Depending on the solvent and on the ratio of these solvents in case of binary mixtures, considerable differences in the retention behavior of these solutes were discovered. Even negative retention could be observed. Using model parameters from single solvent/solute mixtures, a Maxwell–Stefan model based on the assumption of a solution–diffusion transport mechanism was used to predict the solvent as well as the solute fluxes in multi-component mixtures. A good agreement between experimental data and model could be achieved with pure solvent fluxes and solvent fluxes in binary mixtures. However, predictions of solute retentions in solvent mixtures based on solutes/single solvent systems were insufficient and indicate a richer transport complexity in the binary solute/solvent/membrane system. [Copyright &y& Elsevier]