Mechanistic aspects of ethanol steam reforming on Pt, Ni, and PtNi catalysts supported on gamma-Al2O3 are investigated from the analysis of adsorbed species and gas phase products formed oil catalysts (hiring temperature-programmed desorption of ethanol and during ethanol steam reforming reaction. DRIFTS-MS analyses of ethanol decomposition and ethanol steam forming reactions show that PtNi and Ni catalysts are more stable than the Pt monometallic counterpart. Ethanol TPD results oil Ni, Pt, and NiPt catalysts point to ethanol dehydrogenation and acetaldehyde decomposition as the first reaction pathways of ethanol steam reforming over the studied catalysts. The active sites responsible for the acetaldehyde decomposition are easily deactivated ill the first minutes on-stream by carbon deposits. For Ni and PtNi catalysts, a second reaction Pathway, consisting in the decomposition of acetate intermediates formed over the surface of alumina support, becomes the main reaction pathway Operating Ill Steam reforming of ethanol once the acetaldehyde decomposition pathway is deactivated. Taking into account the differences observed in the mechanism of ethanol decomposition, the better stability observed for PtNi catalyst is proposed to be related with a cooperative effect between Pt and Ni activities together with the enhanced ability of Ni to gasify the methyl groups formed by decomposition of acetate species. On the contrary, monometallic catalysts are believed to dehydrogenate these methyl groups forming coke that leads to deactivation of metal particles.