The interpretation of C1s XPS spectra from disordered oxygenated carbons remains uncertain despite a variety of schemes reported in the literature. Here, a thermoseries of cellulose chars was studied to evaluate six published deconvolution schemes; however, none were capable of correctly identifying the oxygen content determined by the O1s spectrum. To improve the self-consistency of the XPS interpretation a method is proposed based on a 7 peak C1s deconvolution, 3 Csingle bondC peaks, 3 oxygenated peaks, and pi-pi* transition peak. Deconvolution of the O1s by 4 peaks is used to determine Osingle bondC and Odouble bondC contributions which provide upper and lower bounds for the related C1s peaks: Csingle bondO, Cdouble bondO and COO. To improve assignments, various functional groups and carbon structures have been examined via DFT using an initial state approximation. DFT calculations of model compounds (pyrene, cellobiose and peryelene tetracarboxylic dianhydride (PTCDA)) were compared with experimental results to confirm the validity of the calculation method used. The DFT calculations identified several defect structures that justify the use of 3 peaks for deconvolution of the Csingle bondC region of C1s XPS spectra. The deconvolution method proposed provides C:O ratios in good agreement (within 5%) of those obtained from total C1s and O1s peaks.