It is important to have an in-depth understanding of the relationship between coal functional group and mass changes during low-temperature oxidation. This knowledge is necessary not only for detecting and preventing the spontaneous combustion of coal but also for reducing emissions of hazardous gases. Microscopic functional group changes of different classes of coal oxidation at temperatures below 230 degrees C were obtained from realtime measurements of the coal matrix via Fourier transform infrared (FTIR) spectroscopy equipped with in situ reactor cells. Mass changes solely due to oxidation between coal and oxygen were obtained by subtracting the TG-N-2 curve from the TG-air curve. Experimental results show that the amount of carbonyl compounds exhibits a linear relationship with the coal mass. Based on two typical reactions of hydrogen elimination and oxygen addition involved in the oxidation process, the relationship between the changes in coal mass and in microscopic functional groups was explored. Experimental results show that with the increase in oxidation temperature, increasing the coal mass via oxygen -containing functional groups becomes more difficult, i.e., more oxidization events are required to increase the same unit of coal mass. The computational formula to find the amount of reactive hydrogen atoms in the coal matrix involved in low temperature oxidation was derived, and the amount of transferable active hydrogen in the three coals was calculated by this formula. This method for finding the amount of transferable active hydrogen determined by TG-FTIR may be promising for assessing the likelihood of coal spontaneous combustion. (C) 2017 Elsevier B.V. All rights reserved.