A systematic study has been made on the desulphurisation of organic sulphur in both feed and mercury-treated coal for their unoxidised samples as well as their oxidised samples prepared at 50, 100 and 150 degreesC in air under normal atmospheric pressure. A continuous increase of organic sulphur loss has been observed; the magnitude of which is substantial in mercury-treated samples. Formation of oxidised organic sulphur functionalities (S=O and -SO2) are identified by infrared spectroscopy. The band in the region 1080-1020 cm(-1) assignable to S=O stretch for all oxidised sulphur species (sulphoxide, sulphone, sulphonic acid, sulfenic acid and sulphinic acid) while the band in the region 1350-1320 cm(-1) attributable to assymmetric and 1170-1120 cm(-1) to symmetric for -SO2, stretch for sulphones, sulphonic acid and sulphinic acid, increase in intensity up to 100 degreesC which decline at 150 degreesC. The absorbance of these bands are qualitatively correlated with those of aliphatic and aromatic C-H content. The aliphatic C-H content follows similar pattern as observed for the absorbance for S=O and -SO2 strech; however, the trend is exactly similar with the mercury-treated sample. The difference of absorbance between S=O and -SO2 gives an indication of the extent of sulphoxide and sulfenic acid formation. Mercury has a catalytic effect in both accelerating the oxidised species formation and their desulphurisation. This study further reveals that up to 50 degreesC desulphurisation is primarily due to the release of free radicals of sulphur species while S=O species along with these radicals predominantly removed beyond 50 degreesC. Knowledge gained from this study would be helpful for various coal conversion processes. (C) 2001 Elsevier Science Ltd. All rights reserved.