Low-temperature oxidation of four different-rank Turkish coals was studied in order to assess the effects of temperature, particle size, coal petrography, and coal rank by monitoring CO2 and CO formation rates and calculated CO/CO2 ratios. Coals were classified to -425 + 212, -212 + 150, and -150 + 106 mu m size groups and oxidized at 40, 60, and 90 degrees C. Temperature has a pronounced effect on the formation rates of CO2 and CO; when the temperature increases, the formation rates of CO2 and CO also increases. Nevertheless, the formation rates of CO2 and CO were found to be independent of particle size, particularly at relatively lower temperatures. Petrographic analysis showed that the coal sample having the highest inertinite group macerals was oxidized more easily, thus, yielding more CO2 and CO. Relatively higher rank coals were oxidized more easily, but oxidation diminishes with time. On the contrary, oxidation progresses with time for lower rank samples especially at relatively higher temperatures. (C) 2011 Elsevier Ltd. All rights reserved.