The chemical nature of oxidized anthracite, obtained by oxidation with air of Bulgarian anthracite at different temperatures was studied by EPR spectroscopy, C-13 NMR solid state, EPR and chemical methods. The X-ray analysis reveals that the oxidation process is confined to the surface of the particles and does not cause considerable change in the internal structure of the material. The C-13 NMR solid state data show that up to 200 degrees C no change occurs in the anthracite composition. Only at 300 degrees C the oxidation process leads to noticeable changes, which become considerable at 400 degrees C. The C-13 NMR spectroscopic data reveal an increase in the amount of the oxygen-containing groups with acid character after oxidation of anthracite at 400 degrees C. EPR investigations show that oxidation of anthracite with air to 300 degrees C does not change the EPR parameters. It was found that the decrease of PMC concentration up to 400 degrees C depends on formation of stable acid oxygen-containing groups, which cannot be removed in the conditions of deep vacuum. The C-13 NMR and EPR data were confirmed by determination of oxygen-containing groups on the oxidation anthracite surface with chemical methods. It was found that a thermooxidation treatment with air at 400 degrees C leads to considerable enhancement of the quantity of oxygen-containing groups on the anthracite surface. These groups were mainly of acidic character and variable chemical nature and have different dissociation strength in aqueous suspension. The pH of oxidized at 400 degrees C anthracite is 4.3.