In this paper, two different crystalline phase compositions appear in the isothermally dried solvent-cast poly(vinylidene fluoride-cohexafluoropropylene) (P(VDF-co-HFP)) membranes, which was explained based on the different solvent-polymer interactions. The dipolar intermolecular interaction between P(VDF-co-HFP) and dimethylacetamide (DMAc) induces polar gamma phase with trans conformation, which is absent in the P(VDF-co-HFP)-acetone system. The different pervaporation behavior of the two solvent-cast membranes in separating ethyl acetate (EtAc) from water was explained based on the different permeant-polymer interactions which were further deduced from the affinity differences. The difference of polarity between EtAc molecules and the acetone-cast P(VDF-co-HFP) membrane is smaller than that between EtAc molecules and the DMAc-cast P(VDF-co-HFP) membrane, which makes for the different Solubilities and diffusivities of EtAc in the two different solvent-cast membranes. And the separation properties of them in pervaporation of EtAc/water mixture are inevitably varied. (c) 2006 Elsevier B.V. All rights reserved.