In this study, a network of reactions involved in the ethanol steam reforming (ESR) over supported Co catalysts has been investigated using temperature-programmed reaction (TPRxn) and isotopic labeling techniques. The products and intermediates in the gas phase have been monitored by mass spectrometry (MS) as a function of temperature. Ethanol conversion and product distribution obtained in steady-state reaction experiments have also been presented. The results point to a complex network of reactions that directly impact the selectivity and H-2 yield that can be obtained in ethanol steam reforming. The reaction network is governed by the temperature range as well as the type of catalyst used, with catalysts exhibiting higher oxygen mobility favoring complete oxidation of ethanol to CO2 and leading to increased hydrogen yield as well as improved stability.