The oxidation of Beypazari lignite was carried out in air at 150-degrees-C for up to 168 h. Oxidized samples were withdrawn from a ventilated oven at different periods of time and characterized by elemental analysis, diffuse reflectance Fourier transform infrared (DRIFT), solid-state C-13 CP/MAS/TOSS NMR, and pyrolysis mass (Py-MS) spectroscopies. It was found that oxidation influenced the organic structure and the inorganics at different periods of the oxidation. Iron(II) sulfate was formed in 24 h, during which time there was almost no change of aliphatic CH2 and CH3 groups. After the formation of iron(II) sulfate, aliphatic CH2 and CH3 groups started to oxidize. Thus, the formation of iron(II) sulfate apparently prevented the oxidation of organic structure of Beypazari lignite during the first 24 h. The van Krevelen diagram (H/C vs O/C), the change of the O/H atomic ratio, and the rate of loss of aliphatic CH2 and CH3 groups showed the organic structure of Beypazari lignite to have continued in three stages. Aromatics seemed to be more stable than aliphatics during the oxidation process. The functional groups produced by the oxidation included ketones, carboxyls, and anhydrides but the major oxidation products were aryl esters. The abundance of CO2+, CO+, CH3COOH+, and SO2+ molecular ions increased in the pyrolysate of the oxidized sample whereas the molecular ions alkylphenols, dihydroxybenzenes, and alklynaphthalenes decreased after the oxidation.