Solubility measurements of the ternary mixture benzene + cyclohexene + cyclohexane in polyurethane (PU) membrane and polyether-block-amide (PEBA) membrane material are reported. The measurements have been performed by swelling membrane samples in the liquid mixtures covering a wide range of ternary compositions. Analysis of the mixture composition in the swollen membrane is made by a gas-chromatographic technique. The results are presented as solubility diagrams revealing a preferred solubility of benzene compared to the other components. A synergistic effect of benzene on the solubility of cyclohexene and cyclohexane is observed particularly in the case of the PU-membrane. The so-called UNIQUAC-model has been applied for predicting the solubility data using only binary interaction parameters obtained from vapor sorption isotherms of the pure solvents in the membranes and from vapor-liquid equilibrium data of the binary mixtures benzene + cyclohexene, benzene + cyclohexane and cyclohexene + cyclohexane. Good agreement between experiments and model prediction is obtained.