Valence‐to‐Core (VtC) X‐ray emission spectroscopy (XES) was used to directly detect the presence of an O−O bond in a complex comprising the [CuII2(μ‐η2:η2‐O2)]2+ core relative to its isomer with a cleaved O−O bond having a [CuIII2(μ‐O)2]2+ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [CuII2(μ‐η2:η2‐O2)]2+ core corresponds to the copper stabilized in‐plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O−O bond activation in μ‐η2:η2‐O2 species and highlight the potential of this method for time‐resolved studies of reaction mechanisms.