This work describes the morphological and material changes in the iron powder during four regeneration-combustion cycles. The regeneration in H2 and combustion in air experiments were made in a fluidized bed (FB) and an entrained flow reactor (EFR), respectively. The average size of the iron oxide particles more than doubled between the first and fourth combustion cycles, and many of the particles were hollow. The regeneration step did not change the size of the particles but increased their porosity. A mechanism is proposed that describes the formation of large-diameter hollow particles which increases as a function of the regeneration-combustion cycles. The observed increase in particle size and the change in particle morphology complicates the iron fuel concept, as it leads to a degradation of the structural stability of the particle with time.