The effect of solvent on properties of solution-cast dense films was investigated using high molecular weight sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) and five different solvents having relatively similar molar volumes. The study revealed that polymer-solvent interactions existing in casting solution primarily determine the concentration of residual solvent and surface morphology of the films. On the other hand, the O-2 and CO2 permeabilities, which for most permeable films were more than three times greater than for the least permeable ones, appear to be governed by the volatility of solvent in casting solution. At the same time, the more permeable films showed lower O-2/N-2 and CO2/CH4 permeability ratios than the less permeable ones. In addition to physical factors such as poly- mer-solvent interactions and volatility of solvent in casting solution, the differences in gas transport properties of SPPO films could arise from the formation of quaternary salts-in particular, in the case of films prepared from the pyridine solution. The analysis of casting solution properties, surface images by atomic force microscopy, and gas transport properties allowed us to associate defective structures of some SPPO films with a specific surface morphology and a particular combination of solvent properties. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1100-1110, 2003.