AbstractSupported vanadia (VO_x) is a versatile catalyst for various redox processes where ceria‐supported VO_x have shown to be particularly active in the oxidative dehydrogenation (ODH) of alcohols. In this work, we clarify the origin of the volcano‐shaped ethanol ODH activity trend for VO_x/CeO_x catalysts using operando quick V K‐ and Ce L₃‐ edge XAS experiments performed under transient conditions. We quantitatively demonstrate that both vanadium and cerium are synergistically involved in alcohol ODH. The concentration of reversible Ce⁴⁺/Ce³⁺ species was identified as the main descriptor of the alcohol ODH activity. The activity drop in the volcano plot, observed at above ca. 3 V nm⁻² surface loading (ca. 30 % of VO_x monolayer coverage), is related to the formation of spectator V⁴⁺ and Ce³⁺ species, which were identified here for the first time. These results might prove to be helpful for the rational optimization of VO_x/CeO₂ catalysts and the refinement of the theoretical models.