A study of the effects of sulfation on the methane oxidation activity of iron oxide catalysts prepared from goethite and 2-line ferrihydrite precursors is presented. Although catalytic performance is found to be dependent upon the precursor, sulfation produces general effects in both systems. Surface area is increased, oxidation activity <400 degrees C is suppressed, and that at 500 degrees C is enhanced leading to the production of selective products. Despite these similarities, sulfation produces different structural effects in the two systems. In the case of iron oxide prepared from goethite, extensive pitting and an increase in disorder in the cationic arrangement are observed. In contrast, a slight increase in crystallinity of the iron oxide prepared from ferrihydrite occurs. The effect of sulfation on catalytic performance is interpreted in terms of a two-site mechanism, complexation poisoning Fe3+ sites active for the lower-temperature oxidation activity with additional sites active at higher temperature being produced.