The effect of zinc promotion on the oxidation state of cobalt in Co/ZrO2 catalysts was investigated and correlated with the activity and selectivity for ethanol steam reforming (ESR). Catalysts were synthesized by applying incipient wetness impregnation and characterized by using Brunauer–Emmett–Teller (BET), temperature-programmed reduction (TPR) measurements, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Higher ethanol conversion and lower CH4 selectivity are observed for the Co/ZrO2 catalyst promoted with Zn as compared to the Co/ZrO2 catalyst alone. Addition of Zn inhibits the oxidation of metallic cobalt (Co0) particles and results in a higher ratio of Co0/Co2+ in the Zn-promoted Co/ZrO2 catalyst. These results suggest that metallic cobalt (Co0) is more active than Co2+ in the ethanol conversion through dehydrogenation and that Co2+ may play a role in the CH4 formation. TPR measurements, on the other hand, reveal that Zn addition inhibits the reduction of Co2+ and Co3+, which would lead to the false conclusion that oxidized Co is required to reduce the CH4 formation. Therefore, TPR measurements may not be appropriate to correlate the degree of metal reducibility (in this case Co0) with the catalyst activity for reactions, such as ESR, where oxidizing conditions exist.