This work reports the results of a study of the thermodynamic swelling behaviour of polar polymeric membranes in contact with purely organic liquid mixtures of strong polarity, The sorption of two aprotic/protic liquid mixtures ETBE/EtOH and AcOEt/EtOH in polyurethaneimide block copolymers was investigated over the entire activity range at 30 degrees C. The polymers used were built from the same pyromellitimide hard blocks and oligomeric segments of different chemical natures (polyether: PTMG, PPC; polyester: PCL) and led to strongly non-ideal ternary systems. The sorption behaviour in these systems revealed particular features that cannot be easily anticipated from the vapour sorption isotherms obtained for the related binary systems, The occurring of strong sorption synergy effects, characterised by sorption coupling factors, led to very significant modifications of the polymer swelling to the detriment of the sorption selectivity. The strongest synergy effects were observed for the swellings of the PUI membranes in ETBE/EtOH mixtures. A comparative study of the semi-predictive modelling of the ternary system behaviours was described on the basis of the properties of the related binary systems and UNIQUAC or derived models. Accurate descriptions of the partial isotherms were obtained for most of the ternary systems investigated but the only system where sorption synergy effects were of particularly great importance.