A precipitated, doubly promoted, iron oxide catalyst was studied to elucidate phenomena that may lead to catalyst attrition during slurry phase Fischer-Tropsch synthesis. The catalyst was examined by electron microscopy (SEM and TEM), electron and X-ray diffraction, sedigraph particle size analysis and BET surface area measurements. The catalyst undergoes attrition both at the micro- as well as the nano-length scales. On the micro-scale, this involves a breakup of ca. 30 mu m spherical agglomerates into ca. 1 mu m particles, a process that can be initiated even by the mixing that occurs in the sedigraph analyzer. On the nano-scale, we find that exposure of the catalyst to CO at increasing temperatures transforms single crystals of alpha-Fe2O3 into Fe3O4 and ultimately to Fe-carbide, with rounded particles as small as ca, 20 nm. The details of phase transformations and the resulting crystallite morphology and size distribution could play a major role in influencing the overall attrition resistance of precipitated iron oxide catalysts. In this paper, we describe the effects of the CO activation temperature on catalyst structure at the micro- and nano-scales.