The structure of Mo at the surface of Fe2O3/SO4 and ferrihydrite catalysts was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. Molybdenum is found to be surrounded by 3-4 oxygens at 1.73-1.75 angstrom with no extended coordination shells beyond the nearest oxygen shell. X-ray diffraction results for the Mo/ferrihydrite catalysts after high temperature annealing demonstrate that the incorporation of Mo at the ferrihydrite surface effectively prohibits crystal growth and allows the catalyst to maintain its dispersion to higher temperatures.