Ammonia synthesis catalyst has been examined by optical microscopy and transmission electron microscopy. In reduced magnetite grains four different pore morphologies with characteristic crystallographic orientations exist. These structures are several micrometers in size and may be described as porous single crystals of bcc-iron. The dominant structure (roughly 80% of the catalyst volume) contains a nearly random network of pores and the crystallographic orientation of the porous iron is the same as that of the original magnetite. Another structure (ca. 5% by volume) has sheets of iron along the three {100} planes of the former magnetite and a [100] axis of iron normal to the sheets. A third structure (ca. 3%) with weakly sheet-like pores along the twelve {112} planes of the original magnetite has a [221] direction of iron normal to the pores. The most complex structure (very rare in the industrial catalyst) displays strongly sheet-like pores along the four {111} planes of magnetite and has a [100] axis of iron normal to the sheets. The particular orientation relations between the original magnetite and the resulting iron suggests that epitaxy plays an important role during reduction. Alumina, which is added as a promoter, reduces the lattice parameter of magnetite. The local Al3+ concentration in the magnetite may influence which structure occurs by changing the likelihood of the possible epitaxy relations. Differences in activity among the various structures may exist. However, activity measurements are not a part of this structural study of the porous iron ammonia synthesis catalyst.