The study was carried out to investigate the effect of thermal exposure on the chemical structure of humic acids with emphasis on the acidic functional groups (carboxylic and phenolic acid groups). This was carried out by treatment of humic acid samples at different temperatures ranging from 200 to 1000 degrees C under nitrogen atmosphere. Oxidation of the coal used followed by the humic acids' extraction led to the selective production of more carboxylic acid groups than phenolic groups although the amount of phenolic groups was found to be higher than the amount of carboxylic groups for both the parent coal and the extracted humic acids. The behaviour of acidic functional groups with the increase in temperature was monitored through analysis of the functional groups using surface-dependent techniques as well as IR. The results obtained from both IR and the surface-dependent functional groups' determination indicated a decomposition of the different functional groups with increase in temperature with carboxylic acids decomposing at lower temperatures (200-400 degrees C), and phenolic groups at much higher temperatures (about 600 degrees C and above). An increase in aromaticity with increase in temperature possibly due to char formation was also observed. (c) 2006 Elsevier B.V. All rights reserved.