An investigation was carried out on the physical and structural transformations occurring during oxidative stabilization of viscose rayon precursor fibers impregnated with diammonium hydrogen phosphate (DAP). Structural characterization was performed using optical microscopy, X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy techniques. The results indicated that the use of DAP impregnation improved the thermal stability of viscose rayon fibers prior to carbonization and activation steps. Analysis of the equatorial X-ray diffraction traces demonstrated gradual loss of crystalline structure due to the disordering processes caused by the interruption of intermolecular hydrogen bonds. A novel X-ray stabilization index for the evaluation of oxidative stabilization was devised specifically for the oxidized viscose rayon fibers. The results obtained from DSC and TGA measurements demonstrated that there was an improvement in the thermal stability due to the formation of an increasing amount of ladder-like structures containing aromatic entities with progressing temperature. The results obtained from the analysis of IR spectra showed the gradual and continuous loss of intramolecular and intermolecular hydrogen bonds associated with dehydrogenation and dehydration reactions. IR spectra demonstrated the loss of crystallinity with progressing temperature confirming the results obtained from X-ray diffraction measurements. IR spectra also demonstrated the formation of double bonded C= C bonds attributed to the formation of a crosslinked ladder-like structure. DSC and TGA measurements show that the sample oxidized at 250 C possesses high thermal stability which can be used for the subsequent carbonization and activation steps. (C) 2013 Elsevier B.V. All rights reserved.