The source of the problem: Experiments with isotopically labeled water can be used to unambiguously assign the source of oxygen in a nonheme manganese(IV) oxo complex. The complex, which was generated using water as an oxygen source and cerium(IV) as an oxidant (see picture), shows reactivities in the activation of CH bonds of alkyl‐functionalized aromatic molecules and the oxidation of aromatic substrates (sub) and benzyl alcohol.