The amount and degree of graphitic character of the carbon deposits formed on Fe-Ni (1:4) particles when heated in an ethane/steam environment at temperature ranging from 765 to 925 degreesC was found to be dependent upon the concentration as well as the chemical nature of the sulfide species present in the reactant stream. Distinct advantages with regard to inhibition of carbon deposition were found when either dimethyl sulfide (DMS) or dimethyl disulfide (DMDS) were used as additives to the reactant mixture compared to the behavior observed with H2S. The introduction of 25 ppm of the organic sulfides produced a significant decrease in the formation of catalytically generated carbon and a corresponding increase in the yield of the desired product ethylene. Under the same conditions H2S was found to promote the growth of catalytic carbon at the expense of ethylene production. Electron microscopy examinations showed that the addition of sulfur species to the reactant feed created a major change in the structural characteristics of the solid carbon deposit as the temperature was progressively raised from 765 to 925 degreesC. At temperatures in excess of 865 degreesC there was a transformation from filamentous to graphitic shell-like structures. The ramifications of this phenomenon on the operation of commercial reactor systems is discussed. (C) 2000 Published by Elsevier Science B.V.