Industrial ammonia synthesis catalyst, in its fresh, reduced + passivated and used + passivated conditions, was studied by temperature-programmed reduction (TPR). The results indicate that about 10% of the total iron atoms or about 5 atomic layers from the surface are reoxidized during the passivation treatment. This is of the same order as estimated by others from gravimetric, microcalorimetric and Mossbauer studies. Reductions after exposure to oxygen at 20-degrees and 100-degrees-C are comparable, but at 200-degrees-C and above bulk oxidation sets in. The TPR of a passivated catalyst after five years of operation in the plant is still similar to that of the fresh catalyst after the first reduction and passivation, but the former has apparently only 2/3 of the iron area of the latter.