Two kinds of high sulphur content coals Mequinenza and Illinois No. 6 have been subjected to reductive and non-reductive alkylation. The reductive alkylation was performed in the potassium/liquid ammonia system, while the non-reductive one was performed according to the procedure proposed by Liotta. In both processes the alkylating reagent was iodomethane. The products were subjected to elemental analysis and studied by IR spectroscopy and Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR) technique. It has been established that much more information can be elicited from AP-TPR results obtained for the samples after modification by reductive or non-reductive alkylation than for non-modified coals. The Mequinenza coal has been found to contain aryl thiols and almost no alkyl thiols, whereas the Illinois No. 6 coal has been shown to contain alkyl thiols while practically no presence of aryl thiols has been detected. (C) 2003 Elsevier Science Ltd. All rights reserved.