Sulfur K- edge X- ray absorption spectroscopy ( XAS) was used to characterize the similar to 0.1% sulfur found both in native reticulated vitreous carbon ( RVC) foam and in RVC oxidatively modified using 0.2 M KMnO4 in 2 M H2SO4. Sulfur valences and functional groups were assessed using K- edge XAS spectral curve-fitting and employing explicit sulfur compounds as models. For native RVC, these were episulfide (similar to 3%), thianthrene (similar to 9%), disulfide (similar to 10%), sulfenate ester (similar to 12%), benzothiophene(similar to 24%), N, N'- thiobisphthalimide ( similar to 30%), alkyl sulfonate (similar to 1.2%), alkyl sulfate monoester (similar to 6%), and sulfate dianion (similar to 6%). Permanganate oxidation of RVC diminished sulfenic sulfur to similar to 9%, thianthrenic sulfur to similar to 7%, and sulfate dianion to similar to 1% but increased sulfate monoester to similar to 12%, and newly produced sulfone (similar to 2%) and sulfate diester (similar to 5%). A simple thermodynamic model was derived that allows proportionate functional group comparisons despite differing (similar to +/- 15%) total sulfur contents between RVC batches. The limits of accuracy in the XAS curve- fitting analysis are discussed in terms of microenvironments and extended structures in RVC carbon that cannot be exactly modeled by small molecules. Sulfate esters cover similar to 0.15% of the RVC surface, increasing to similar to 0.51% following permanganate/ sulfuric acid treatment. The detection of episulfide directly corroborates a proposed mechanism for the migration of elemental sulfur through carbon.