The microstructural changes occurring on the surfaces of magnetite samples during the reduction in H/N gas mixtures have been directly recorded using an optical heating stage technique. At low temperatures, below approximately 910 K, the iron forms a porous morphology and grows in a continuous manner at a rate dependent on the reaction temperature and hydrogen partial pressure. At high temperatures, iron growth occurs through an unusual discontinuous mechanism, and at any given hydrogen partial pressure, the growth exhibits a rate minimum with increasing reaction temperature.