Thermogravimetry (TG), in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) and on-line mass spectrometry (MS) were combined to study the effect of O-methylation and O-acetylation on the hydrogen bonds (HBs) in coal. A new method was proposed to compare the spectra of coal before and after the treatments by calibration of the band of kaolinite in coal. It was shown that both measures could reduce the amount of HBs in coal significantly (particularly the weak HBs), but neither of them could completely eliminate the HBs in coal because of the spatial resistance of three-dimensional structure in coal. It was proved that the stronger the hydrogen bonds formed by -OH groups, the less the extent to which they were reduced. The effect of O-acetylation is stronger than that of O-methylation on the reduction of HBs due to the low penetrability of the solvent used in the later treatment. Compared with the parent coal, both the pyrolysis temperature range and conversion of the treated coal increased, the initial decomposition temperature lowered, the decomposition rate accelerated and the composition of gas products was improved. Among the gases produced from the treated coal with these two methods, the amount of C-1 - C-4 hydrocarbons increased, and CO2 and water reduced during pyrolysis compared with those of the raw coal. (C) 2004 Elsevier B.V. All rights reserved.