Thermal stabilization of polyacrylonitrile (PAN) precursor fiber was performed with a pretreatment of an aqueous guanidine carbonate solution and its structure was thoroughly characterized using a combination of infrared spectroscopy (IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), density, elemental analysis, and X-ray diffraction measurements. The use of guanidine carbonate pretreatment of polyacrylonitrile precursor fiber was found to be very useful for the acceleration of thermal stabilization of polyacrylonitrile precursor fiber prior to the carbonization stage. The results obtained from density, thermal analysis (TGA and DSC), infrared-spectroscopy and X-ray diffraction methods suggested an accelerated thermally stable aromatic ladder structure formation resulting in much reduced thermal stabilization time. X-ray observations showed the transformation of the original structure from a highly ordered phase to a totally disordered amorphous phase which seemed to be a direct consequence of the crosslinked and cyclized structure present in the stabilized fibers. The results obtained from the infrared spectra of thermally stabilized samples showed a rapid and simultaneous cyclization and dehydrogenation reactions aided by the oxygen uptake in the form of oxygen containing functional groups. Guanidine carbonate pretreated and thermally stabilized PAN precursor fibers showed a carbon yield of 52.5% at 1100 degrees C obtained from TGA measurements. The use of guanidine carbonate pretreatment is expected to significantly increase the productivity of carbon fiber manufacturing at a substantially reduced cost by significantly reducing the time necessary for thermal stabilization of polyacryonitrile fiber. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers