Novel Co-MnO and CoCr2O4 catalysts were found to be tolerant to sulfur up to levels of 220 ppm under WGS reaction conditions. However in coal-derived process gas containing between 0.25 and 0.3 mol% sulfur, and at a reaction temperature of 400 degrees C the Co-MnO catalyst deactivated rapidly and irreversibly with formation of bulk Co9S8 and a surface manganese sulfide species. The CoCr2O4 catalyst partially deactivated under similar conditions, a phenomenon related to the formation of a surface sulfate species. Bulk sulfiding of the CoCr2O4 catalyst to CoCr2S4 occurred at 550 degrees C and this catalyst gave near-equilibrium CO conversions in the WGS reaction. A pre-sulfided cobalt chromium catalyst demonstrated typical sulfur dependent mechanistic characteristics, with a maximum activity above 400 degrees C.