The reformer sponge iron cycle (RESC) process was introduced for the stationary, decentralised production of hydrogen from hydrocarbon-containing fuels. The RESC process consists of two steps: the reformation of higher hydrocarbon to synthesis gas and the fine purification of this gas to pure hydrogen with the sponge iron reaction (SIR) process. The SIR process uses iron ore as contact mass. The contact mass (iron oxide) is reduced to iron in the first cycle by a synthesis gas, and is re-oxidised into iron oxide in the second cycle, utilizing steam. Pure hydrogen is produced in the second cycle as reaction product of the process. Iron ore is a very inexpensive base material for the contact mass, but the contact mass still has to be stable over several thousand redox cycles. Test series with varying contact mass compositions have been performed in order to investigate the influence of the composition on the durability of the contact mass. Carbon monoxide and hydrogen were used for the reduction process. Thermogravimetry (TG), X-ray diffractometry (XRD), scanning electron microscope (SEM) and mercury porosimetry were applied for the evaluation of structural changes after cycling the contact mass. The results confirm the importance of the skeletal structure of the pellets. (c) 2005 Elsevier B.V. All rights reserved.