The role of Sm2O3 and CeO2 promoters on the structural properties and catalytic behavior of Rh/xSm(2)O(3)-yCeO(2)-Al2O3 catalysts during methane steam reforming (MSR) was investigated. Promoted catalysts showed higher reaction rates per surface Rh atom and improved stability compared to Rh/Al2O3. In situ X-ray absorption revealed that the structure of Rh particles in Rh/Al2O3 changes drastically during MSR, while it was stable in the presence of Sm2O3-CeO2. Sintering of the active metal phase was the main cause of deactivation. STEM images showed stronger Rh agglomeration of the unpromoted catalyst with time on stream. (C) 2012 Elsevier Inc. All rights reserved.