Characterization of the crosslinked structure has been carried out for soft polyimide gels prepared from a soluble high-performance fluorine-containing polyimide consisting of 4,4＇-(hexafluoro-isopropylindene)diphthal anhydride and 3,3＇-dihydroxy-4,4＇-diaminobiphenyl crosslinked with hexamethylene diisocyanate. Weight-average molecular weight (M-W = 9.28 x 10(4)), second and third virial coefficients (A(2) = 0.82 x 10(-3) mol ml/g(2), A(3) = 6.42 x 10(-3) mol ml(2)/g(3)) of the linear polyimide in N-methylpyrrolidone at 24 degrees C were determined by a static laser light scattering measurement. The average molecular weight between crosslinking points M-c, was calculated based on both swelling and viscoelastic measurements of the polyimide gels. As for the equilibrium swelling data, a concentration-dependent form of the Flory-Huggins parameter (i.e. chi = chi(1) + chi(2) phi) has been applied to calculate the M-c. The values of M-c from equilibrium swelling and viscoelastic measurement have been found to be in agreement. By comparing M-c with the results of H-1-NMR, the percentages of effective and ineffective linkages have been evaluated quantitatively for all the polyimide gels. It has been found that the elastically effective crosslinking is less than 20%. Results from classical and improved Flory-Rehner theory have been compared and we believe that the improved theory gives a more reasonable description of M-c at higher crosslink density. (C) 2000 Elsevier Science Ltd. All rights reserved.