Demineralized (HCl/HF) samples of Beypazari lignite were oxidized in air at 150 degrees C for up 120 h in a ventilated oven. Elemental analyses, diffuse reflectance Fourier transform infrared (DRIFT), solid-state C-13 CP/MAS/TOSS NMR, and pyrolysis mass (PY-MS) spectroscopies as well as solvent swelling were used for the characterization of the oxidized and unoxidized samples. It was found that the removal of 90.1% of the mineral matter from Beypazari lignite facilitated the access of oxygen into the coal structure and permitted the occurrence of diffusion-controlled reactions. This was confirmed by the observation of linear relationships between the decrease in the intensity of aliphatic CH2 and CH3 groups, the increase of the O/H atomic ratio and the decrease in the aliphatic factor, and the square root of the duration of oxidation. It was observed from swelling measurements in pyridine before and after the oxidation that aryl esters and anhydrides, which were the major oxidation products, probably acted as covalent cross-links. Aliphatic groups oxidized more rapidly than aromatic structures and, in fact, methylenes oxidized more rapidly than methyls. The decrease in the intensity of the molecular ion series alkylphenols, alkyldihydroxybenzenes, alkylbenzenes, and alkylnaphthalenes arising from pyrolysis mass spectrometry of oxidized samples was approximately consistent with the corresponding increase in the intensity of CO2+, CH3COOH+, and CO+ molecular ions.