Mossbauer data indicate that activating an iron Fischer-Tropsch catalyst containing Cu, K, and kaolin with syngas for 24 h at 270 degrees C and 1.14 MPa produces a material that has about 60% of-the iron in the form of Fe3O4 and the remainder as a mixture of chi-Fe3C4 and epsilon’-Fe2.2C. A similar activation with CO produces a material that contains more than 80% of the iron in the form of chi-Fe5C2 and the remainder as Fe3O4. The CO-activated catalyst exhibits a high activity, whereas the syngas-activated sample has a low activity. Moreover, the CO-activated sample converts during 50 h of synthesis to a mixture that contains more Fe3O4 (40%) and less chi-Fe5C2 (60), whereas the syngas activated sample contains about 80% chi-Fe5C2 after 50 h of exposure to synthesis gas. Thus, both catalysts underwent significant changes in the bulk composition without a parallel change in CO conversion. Another catalyst formulation, containing more Cu, much less K, and no binder, showed significantly higher syngas conversion after activation by CO than by syngas even though the bulk phase compositions were similar. A catalyst, initially activated with syngas and exhibiting low activity, showed evidence for only Fe3O4. This low-activity material after a 24 h treatment in CO exhibited a dramatic increase of the carbide phase and catalytic activity. The analysis of phase composition is informative about changes in the bulk composition of iron catalysts, but these changes do not necessarily track directly catalytic activity.