Copolyimides were synthesized in N-methyl-2-pyrrolidone (NMP) to prepare polyimide (PI) solutions with various concentrations, and gels can form in PI/NMP solution with a suitable concentration or at a low temperature. Gel-sol transition in the PI/NMP solution has been investigated through a combination of polarized optical microscope, differential scanning calorimeter measurement, and rheological measurements. According to Winter-Chambon theory, the critical gel point is 12 wt %, and the loss tangent (tan ) shows frequency independence. Gel-sol transition temperature (T-gel) for the 13 wt % solution is around 65 degrees C. Below the T-gel, the gel exhibits strong optical planar texture, indicating the formation of a fully anisotropic phase, which has a great importance for the fabrication of PI fibers by gel-spinning. Dynamic temperature sweep measurements show that the formation of the gel from solution is thermoreversible and is significantly delayed in the cooling process. Under the favorable conditions, the tensile strength and modulus of the fibers derived from the solution are about 0.7 and 12.5 GPa using gel-spinning method, which is believed to afford broad application potential in preparing high-performance PI fibers. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 450-459