The thermodynamics of polyacrylonitrile (PAN) terpolymer/dimethyl sulphoxide (DMSO)/water system was investigated by viscometric method. Fourier transform infrared (FTIR) measurement of the temperature dependence of polymer/solvent interaction was performed in the range of 25-80 degrees C, which was in good agreement with viscometric results. Meanwhile, the upper critical solution temperature (UCST) for PAN terpolymer/DMSO/water system, which is proved to be stable one, was determined from the temperature dependence of the expansion factor alpha(3)(eta). The morphology of PAN precursor prepared by dry-jet-wet spinning with different fiber-forming conditions was examined with a scanning electron microscope (SEM). Judging from SEM photographs, not only the number and size of microvoids of PAN precursor gradually increase, with increasing the temperature of coagulation bath, but also the cross-section shape of PAN precursor changes from nephroid shape to elliptical shape or circular shape. Therefore, PAN precursor with different microstructures can be fabricated at different quenching-depths, suggesting that the final microstructure of the PAN precursor greatly depends on the phase separation in the fiber-forming process. (C) 2008 Wiley Periodicals, Inc.