Structural changes occurred in Beypazari lignite while oxidizing at 50 degrees C, 100 degrees C and 150 degrees C in a dynamic air atmosphere was detected by infrared spectroscopy. Relative changes in the amount of various oxygenated functional groups with respect to time were determined by curve resolution procedures. Investigation of the spectra of the samples at different oxidation conditions reveal that as the duration and temperature of the oxidation reactions increased the absorption in the aliphatic C-H stretching region decreased, while the absorption in the C=O stretching region presented a significant increase.