A polysulfone spinning solution used recently to produce enhanced selectivity gas separation hollow fiber membranes was theologically assessed using a rotational rheometer and an optical shear cell. Effects of temperature and shear rate on viscosity, power law behavior and normal force provided some clues regarding phase inversion and molecular orientation. At relatively low temperatures, phase inversion may occur in the absence of a shear held. At moderately low temperatures, phase inversion may be induced by applied shear. At higher temperatures, phase inversion is not induced by shear but rather shear induces molecular orientation. The results suggest that, unless spinning at low temperature, extrusion shear does not directly induce demixing during membrane formation but, instead, is linked indirectly to phase inversion through induced molecular orientation which, in turn, affects the subsequent dry or ever precipitation stages in spinning. This work is a step towards the construction of phase diagrams and determining their distortion in shear fields. Such knowledge, coupled with deeper insights into induced polymer molecule orientation, would enable further improvements in spinning techniques and membrane performance. (C) 2001 Elsevier Science Ltd. Ail rights reserved.