The mechanism of the oxidation of coal at low temperatures, i.e., below 100degreesC, was examined using measurements of the gases emitted from a bed of coal in an isothermal flow reactor. Employing an online two-column micro gas chromatograph, transient rates of production of CO2 and CO were monitored during desorption and oxidation experiments. A bituminous coal was milled into three nominal top size classes: 0-0.5 mm, 0-1 mm, and 0-2 mm. Desorption experiments with unoxidized coal samples at 20-70degreesC indicated that even an unoxidized coal incorporates oxygenated complexes in its structure. The threshold for thermal decomposition of these oxygenated species was found to be between 50 and 70degreesC. Carbon oxides liberated from oxidizing coal were compared with those from the thermal decomposition of coal oxidized at the same temperature, suggesting that two parallel reaction sequences contribute to the emission of carbon oxides during oxidation. A multi-step reaction mechanism was also proposed to describe low-temperature oxidation of coal and to explain the phenomena observed during the desorption and oxidation experiments. (C) 2003 The Combustion Institute. All rights reserved.