Non-isothermal oxidation enthalpies have been measured for three Colombian coals which have been weathered under ambient conditions for period up to 105 days. It was found that although the total oxidation enthalpies decreased with increasing oxidation the decrease was not systematic. It was thought that the random behaviour was due to weight losses during the course of the experiments. It was noted that the onset temperature of oxidation increased with oxidation in a more systematic way and also increased with increasing coal rank. It was therefore proposed that the onset temperatures was a better indicator of the propensity of the coals to oxidation.