The aim of this work has been to analyze how different solvents used in the manufacture of gas separation membranes, based on 6FDA-6FpDA, influence permeability and selectivity. It has been shown that a low boiling point and/or a repulsive interaction between the solvent and the polymer increase both selectivity and permeability for O-2/N-2 and, specially, for CO2/CH4 gas pairs. These results have been explained in terms of fractional free volume and intersegmental mobility. In particular it has been shown that good solubility of the polymer results in a high glass transition temperature and planar surface as characterised by the fractal dimension of the resulting membranes, obtained from AFM analysis. (c) 2007 Elsevier B.V. All rights reserved.