The effects of adding Co to a Ni/MgAl2O4 catalyst on activity and stability in the oxidative steam reforming of ethanol (OSRE) were investigated. The catalysts were prepared by impregnation and characterized by X-ray diffraction, N-2 physisorption, temperature-programmed reduction, transmission electron microscopy, and X-ray absorption near edge structure (XANES). The major products formed during the OSRE reaction at 500 degrees C were H-2, CO2, CO, and acetaldehyde, together with small amounts of CH4, ethylene, and acetone. The ethanol conversion was close to 100% with selectivity to H-2 around 55% for 4Co4Ni catalyst, and no deactivation was observed on this catalyst during 28 h of reaction. The oxidation and reduction of Ni and Co catalysts during OSRE reaction were studied by XANES. Initial deactivation during the OSRE reaction occurred due to metal oxidation during heating, and the catalysts became reduced again as the reforming reaction proceeded during heating at temperatures higher than about 500 degrees C. Greater addition of Co to Ni and smaller metal particle size favored metal oxidation during OSRE reaction. The presence of metal oxide on the surfaces of the metal nanoparticles promoted the oxidation of carbon on the metal surface, hence decreasing the rate of carbon accumulation. The results indicated that a stable and active catalyst for H-2 production by means of the OSRE reaction could be produced by combining appropriate amounts of Co and Ni and using metal particle sizes smaller than about 5 nm.