Thermal-induced gelation and phase separation was investigated in PAN/DMSO/H2O semidilute solutions utilizing rheological and optical methods. The incipient precipitation temperature, where a distinct decrease in relative light transmittance occurred, was considered as the cloud point (CP). Phase separation mechanism of nucleation and growth was confirmed by various ramp rates experiments and viscoelastic evolution pattern. On the other hand, the validity of Chambon-Winter model was evaluated in these blends and used to measure the critical gel point (GP). It was found that in the accessible temperature regions CP was always higher than GP, and phase boundaries were constructed accordingly. What's more, the absolute complex viscosity eta' at a low frequency also showed transition points during temperature decreasing experiments, which mostly located between CPs and GPs. The gelation mechanisms with and without phase separation were proposed and the effect of the two transitions on the structures of the ternary blends were shown by SEM pictures. (C) 2016 Elsevier Ltd. All rights reserved.