Differentiation of organic sulfur forms in coal by step-wise oxidation with a mixture of perchloric acid (HClO4) and ferric perchlorate (Fe(ClO4)(3)) was studied. Various organic sulfur compounds were oxidized with HClO4 solution containing Fe(ClO4)(3), and the amounts of sulfate formed during reaction were measured. The compounds can be grouped into three categories according to their reactivities as follows. (1) easily oxidized ones, i.e. disulfides, for which almost all of the sulfur was converted to sulfate after oxidation, (2) less reactive ones including aliphatic sulfides, aliphatic and aromatic thiols (20-30% of sulfur in these compounds was oxidized to sulfate), and (3) relatively stable ones, i.e. thiophenes and aromatic sulfides, from which no sulfate was formed. Samples of Illinois No. 6 and Bevier coals were also reacted with HClO4 solutions containing increasing concentration of Fe(ClO4)(3). The organic sulfur in these coals could be differentiated into various groups according to their reactivities.