The activation of molecular oxygen is afundamental step in almost all catalytic oxidation reactions.Wehave studied this topic and the role of surface vacancies for Co 3 O 4 (100) films with as ynergistic combination of experimental and theoretical methods.Weshow that the as-prepared surface is Blayer terminated and that mild reduction produces oxygen single and double vacancies in this layer.O xygen adsorption experiments clearly reveal different superoxide species below room temperature.T he superoxide desorbs belowc a. 120 K from avacancy-free surface and is not active for CO oxidation while superoxide on as urface with oxygen vacancies is stable up to ca. 270 Ka nd can oxidize CO already at the low temperature of 120 K. The vacancies are not refilled by oxygen from the superoxide, whichmakes them suitable for long-term operation. Our joint experimental/theoretical effort highlights the relevance of surface vacancies in catalytic oxidation reactions.