The catalyst's performance and deactivation in a Midrex (R) industrial fixed bed reactor were investigated for the combined steam and dry reforming of natural gas using a one-dimensional heterogeneous model. The results demonstrate that there is a strong tendency for the catalyst's deactivation by carbon formation originating from methane decomposition. However, kinetic modelling of the combined reforming process shows that only a fraction of the catalyst in the industrial reactor is required for the reactions to reach an equilibrium state in the reformer. Hence, as the catalyst is deactivated at the reactor entrance area, the reaction zone gradually moves forward and still allows for equilibriums to be attained before the stream exits the reactor. This results in a rather constant methane conversion in the reformate stream (up to several months) despite the fact that the catalyst is being deactivated layer by layer as the operation continues. X-ray diffraction and N-2 sorption analyses were to investigate the structural variations of the catalyst within the 41-month operational study period of the reforming reactor. Comparisons between the structures of fresh and spent reforming catalysts show an increase in Ni crystal size and a decrease in the surface area of the catalyst as the process continues. (C) 2013 Elsevier B.V. All rights reserved.