The copolyimides derived from 3,4-oxidianiline (ODA), 4,4'-oxydiphthalic anhydride (ODPA) and 3,3',4,4-biphenyl dianhydride (B]PDA) were synthesized in N,N-dimethyl acetamide (DMAc) by a two-step method. To control molecular weight, phthalic anhydride (PA) was used as an end-capping reagent. The effect of far-infrared radiation (FIR) on imidization of copolyimides was investigated by Fourier transform infrared spectroscopy (FT-IR) and Thermogravimetric analysis (TGA), and was compared to conventional electrothermal imidization. The effect of imidization time and film thickness on weight loss of imidization process was also studied. The results show that the FIR can accelerate the imidization process of copolyimides compared with the conventional electrothermal treatment especially during the primary curing period. Weight loss of thin film is faster than that of thick film. In addition, molecular weight will influence the imidization process. (c) 2005 Wiley Periodicals, Inc.