In order to understand the transformation mechanisms of iron-sulfur catalyst systems to pyrrhotite, the iron catalysts (gamma-FeOOH, alpha-FeOOH, Fe(CO)(5) and Fe-3(CO)(12)) and sulfur compounds (S and H2S) were heat-treated at 150-420 degrees C with or without an activated carbon, and then subjected to XRD and XPS analyses. Pyrrhotite (Fe1-xS) was the major phase above 200 degrees C in the XRD profiles of all iron-sulfur catalyst systems. However, the formation of pyrite (FeS2) from gamma-FeOOH and gamma-FeOOH on the catalyst surface was observed at 150-325 degrees C by XPS analyses. This result seems to indicate that active species (Fe1-xS) would be transformed through FeS2 as an intermediate, but iron carbonyl complexes were directly transformed into pyrrhotite without the formation of FeS2. (C) 2000 Elsevier Science Ltd. All rights reserved.