The influence of the addition of 1-10 vol.% of hydrogen or carbon dioxide to the feed during the partial oxidation of methane was studied over a NiO/gamma-Al2O3 catalyst. The addition of H-2 decreases the conversion and syngas selectivity. This decrease of performance seems to be related to a higher reduction of the catalyst due to the H-2 co-feeding. The addition of CO2 also appears unfavorable to the production of hydrogen but increases the CO yield. A combination of the dry reforming and the reverse water gas shift reactions is suggested to explain the observed modifications in the product yields. (c) 2005 Elsevier B.V. All rights reserved.