Mechanism of ethanol steam reforming into syngas over Pt/Pr0.15Sm0.15Ce0.35Zr0.35O2 and 10 wt% LaNi0.9Ru0.1O3/Mg-Al2O3 catalysts was studied by in situ FTIRS and pulse titration experiments. Surface species (ethoxy, adsorbed ethanol, acetaldehyde, acetate, etc.) were identified and their thermal stability, routers of transformation and reactivity were characterized. Acetate species were shown to be spectators for both types of catalysts. Transformation of ethoxy species by dehydrogenation is a fast step, while the rate-determining stage is the C–C bond rupture in thus formed acetaldehyde on metal sites. For Pt/Pr0.15Sm0.15Ce0.35Zr0.35O2 catalyst with a high mobility and reactivity of the surface/lattice oxygen of support, efficient oxidative transformation of acetaldehyde at the metal-support interface provides a high yield of syngas at short contact times in the intermediate temperature range with a minor amount of CH4 by-product. Transformation of ethoxy species on the acid sites of alumina-supported catalyst produces C2H4 and (C2H5)2O via dehydration route dominating at temperatures below 400 °C. In addition, for alumina-supported catalyst acetone is produced via aldol formation in the temperature range 400–500 °C due to combined action of metal and support sites. For this catalyst syngas yield is improved at high temperatures when steam reforming of these byproducts efficiently proceeds accompanied by cracking reactions producing also methane as by-product.