Hydrogen diffusion in single crystals of iron oxides (Nb doped Fe~O3 and FeO) was studied by secondary ion mass spectroscopy on samples deuterated in a RF plasma. Depending on the temperature of the exposure to deuterium, the deuterium profiles are representative of trapping with a concentration near the dopant level or of a nearly "Fickian" diffusion process. The apparent diffusion coefficients confirm that deuterium (hydrogen) diffusion is slow in the iron oxides.