Ultrafine nanometer-scale iron-based catalyst precursor powders were generated using two novel technologies, the rapid thermal decomposition of precursors in solution (RTDS) and the modified reverse micelle (MRM) processes. The powders were characterized according to their phase and crystallite size and were evaluated for activity toward C-C bond scission using the model compound, naphthylbibenzylmethane, in the presence of elemental sulfur and 9,10-dihydrophenanthrene. The catalytic activities of the powders were found to be strongly dependent on their crystallographic phase. RTDS magnetite, six-line ferrihydrite, and ferric oxyhydroxysulfate were found to have very high activity toward conversion of the model compound whereas two-line ferrihydrite and hematite were determined to be poor or mediocre catalyst precursors. MRM magnetite/maghemite was also found to be a relatively good catalyst precursor but exhibited reduced activity when compared to the RTDS magnetite.