In the steam-iron process pure hydrogen can be produced from any hydrocarbon feedstock by using a redox cycle of iron oxide. One of the main problems connected to the use of the iron oxide is the inherent structural changes that take place during oxygen loading and unloading leading to severe deactivation. This deactivation reduces the capability of the material for uptake and release of oxygen, basically due to loss of specific surface area. In this paper a simplified (reactive) sintering model is used to derive a relation for the loss in surface area of the material in the first redox cycles. This model is based on the relative conversion and the resulting swelling of the material during oxidation. Furthermore the grainy pellet model is used to describe the increase in grain size and increase in solid fraction in a particle due to the subsequent cycling of the iron oxide in the redox process. Model predictions are compared with redox experiments of H-2/H2O-Fe/Fe3O4 at 800 degrees C. Grain growth over subsequent cycles could explain the observed deactivation over about 20 cycles satisfactorily. (C) 2009 Elsevier B.V. All rights reserved.