The influence of the addition of 5 vol.% of carbon monoxide, hydrogen, carbon dioxide or water to the feed of partial oxidation of methane was investigated over Ni/gamma-Al2O3 and Rh/gamma-Al2O3 catalysts. In addition to catalytic tests, thermodynamic calculations were performed to predict the effect of these gas co-feeds. Compared to the thermodynamic trends, differences in the influence of the co-feeding on catalytic performances were observed between both catalysts. Co-feeding of CO, H-2, CO, or H2O can modify the oxidation state and dispersion of the metal component of the catalysts during reaction, and as a consequence, their performances. Changes in catalysts can be due to dynamic processes occurring during reaction. It is suggested to take these processes into account in a more complex kinetic equation for the reactions involved. (c) 2005 Elsevier B.V. All rights reserved.