Abstract The catalytic activity of sulphate found in S-promoted iron oxide catalyst was investigated using model compounds such as diphenylether and diphenylmethane. The formation of sulphate was carried out by addition of water in the S-promoted iron oxide system. The catalysts used were FeS 2, Fe 2 O 3/ S, Fe 3O 4 S , red mud S and FeSO 4 of analytical reagent grade. Reaction was carried out in the absence and presence of water under H 2 or N 2 atmosphere. When diphenylether was reacted under H 2 atmosphere, reaction products were found to be benzene and phenol, but unknown compounds with high boiling points were also found under N 2 atmosphere. Diphenylmethane was converted to benzene, toluene, bibenzyl, triphenylmethane, anthracene and unknown compounds under H 2 and N 2 atmosphere. It was shown that sulphate clearly promoted radical formation from phenyl and benzyl groups by dehydrogenation and the radicals formed were polymerized to various higher volatile compounds under N 2 atmosphere as well as under H 2 atmosphere, atmosphere.