The oxidative thermal stability of microcrystalline plant-based viscose rayon cellulose fiber was improved in the presence of an ammonium sulfate solution. The structural characterization of the oxidized samples was carried out using a combination of fiber thickness, linear density, burning test, X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy measurements. Oxidized viscose rayon fibers showed physical and chemical transformations with increasing oxidation temperature and were characterized by a reduction in fiber diameter and linear density values together with color changes with increasing oxidation temperature. The results obtained from XRD analysis showed the loss of crystallinity due to the amorphization processes. XRD analysis also showed increasing conversion to a highly crosslinked and cyclized structure with progressing temperature. The DSC and TGA measurements indicated that there was an improvement in thermal stability. The results obtained from IR spectra showed the progress of dehydrogenation and dehydration reactions. IR spectra in the 2000-800 cm(-1) region showed a loss of crystalline structure confirming the results of XRD. The IR spectra of the oxidized samples also showed the conversion of primary hydroxyl (OH) groups to carbonyl (C=O) groups and the presence of C=C double bonds arising from the formation of a cyclized and crosslinked structure. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 1239-1249, 2013