The microstructures of fused iron catalysts in the unreduced and partially reduced state have been investigated using analytical electron microscopy techniques. In the unreduced state, B "’-potassium ferrites of 0.1 to 0.3 mu m in size were found to precipitate uniformly and semi-coherently at the centers of the magnetite dendrites. These precipitates were identified as K2O . 4FeO . 15Fe(2)O(3) potassium ferrites, having lattice fringes of ca. 1.72 nm, which corresponded to the distance between the [KO](-) planes. During hydrogen reduction, a well delineated porous structure develops which can be described by the (0001)K2O . 4FeO . 15Fe(2)O(3) parallel to (($) over bar 111)Fe3O4 (($) over bar 110)Fe parallel to Pore Channel and the (2 ($) over bar 1 ($) over bar 10)K2O . 4FeO . 15Fe(2)O(3) parallel to (0 ($) over bar 22)Fe3O4 parallel to (002)Fe perpendicular to Pore Channel, crystallographic orientation relationship. This indicates that the potassium ferrite precipitates, which are still present in the partially reduced structure, have their potassium conduction bands parallel to the pore channels. It was concluded that the presence of these uniformly distributed B "’-potassium ferrites and their orientation relationship with the pore channels are responsible for the high atomic surface coverage of potassium obtained in the catalyst.