The effects of the quaternary ammonium group size and the solvent nature and composition on the viscometric behavior of several quaternized polysulfones (PSFs) were investigated. The solution viscosity of the cationic PSFs was determined over a wide concentration range in N,N-dimethylformamide (DMF). The experimental data were plotted in the terms of the Fedors equation. It was found that the intrinsic viscosity values were strongly dependent on the size of quaternary ammonium groups. The viscosity of PSF quaternized with N,N-dimethylbutylamine (PSF-DMBA) was also measured in the mixed solvents consisting of DMF/methanol and DMF/diethylether. Polyelectrolyte behavior was observed for PSF-DMBA in the mixtures of DMF/methanol for all compositions employed and in the mixtures of DMF/diethylether with the diethylether content as high as 25 vol %. The viscosity decrease and precipitation in the polymer solution with increasing diethylether content was attributed to the dipole-dipole attraction between ion pairs formed in a less polar medium. (C) 2003 Wiley Periodicals, Inc.