Beypazari lignite was subsequently demineralized with HCl/HF and extracted by pyridine under supercritical conditions. The extracted residue was oxidized in air at 150-degrees-C for up to 120 h in a ventilated oven. Elemental analysis, diffuse reflectance Fourier transform infrared (DRIFT), solid-state C-13 CP/MAS/TOSS NMR, and pyrolysis mass (Py-MS) spectroscopic techniques were used for the structural characterization of the oxidized and unoxidized samples. It was found that the oxidation of the extracted residue proceeded in two consecutive stages. The rate of oxidation, as measured by changes in the concentrations of carboxyl and aliphatic CH2 and CH3 groups and in the O/H atomic ratio, was much faster in the initial than in the second stage. The removal of 90.1% of the mineral matter and 39.7% of the soluble material from the parent lignite facilitated the access of oxygen to the network structure which was relatively enriched in aromatic units by the supercritical fluid extraction. It was therefore easier to form aryl esters, alkyl esters, and anhydrides. Since aliphatic structures were relatively less in concentrations than aromatics, there was little change in the intensities of alkylbenzenes, alkylnaphthalenes, alkyldihydroxybenzenes, alkylphenols, and alkanes during oxidation. The higher intensities of CO2+ (m/z 44) and CH3COOH+ (m/z 60) molecular ions observed in Py-MS than CO+ (m/z 28) molecule ion indicated that more carboxyl-containing functional groups were formed by oxidation than carbonyl-containing groups. Aromatic structures in the organic network were not affected during oxidation. The oxidation pathway of the extracted residue was considered to lie between of the oxidation pathways of the parent and the demineralized Beypazari lignites.