The sorption characteristics of benzene and cyclohexane vapours in the poly(urethane-urea) (PUU) with well defined structure were measured using an improved version of the Mc Bain＇s microbalance. The diffusion coefficients were calculated for the different vapour activities from the kinetics of solvent uptake, i.e. the sorption isotherms over time. Sorption and diffusion coefficients measured, showed a strong dependence of the applied vapour activity. The obtained transport parameters were discussed with regard to the polymer structure. Based on the sorption and diffusion data, the pervaporation performance of the PUU-based membrane in separation of the benzene/cyclohexane mixtures was predicted and compared with the experimental pervaporation characteristic.