Viscosities eta were measured for moderately concentrated (15.5 wt%) solutions of poly(ether imide) (PEI) as a function of the composition of the cosolvent tetrahydrofuran (THF) plus gamma-butyrolactone (gamma-BL). The solutions are almost by a factor of four less viscous when THF predominates than when gamma-BL, is the major component of the solvent mixture. Furthermore, the viscosity passes a minimum as the composition of the mixed solvent is varied. Both observations can be well understood in terms of the viscosities of the mixed solvents and the hydrodynamic interaction between the polymer molecules. The pronounced dissymmetry of eta as a function of the composition of the mixed solvent eta results from the fact that the viscosity of gamma-BL is three times larger than that of THF. The minimum in eta reflects the dependence of the thermodynamic quality of the mixed solvent on its composition. With increasing predominance of one component the solvent deteriorates and additional polymer/polymer contacts-leading to an augmentation of eta-are formed. (C) 2000 Elsevier Science Ltd. All rights reserved.