We find that the dissociation of O • - 2 from Cu(II) in 2 is feasible but energetically quite uphill. However, if O • - 2 remains at the active-site, the following proton transfer from the second-sphere histidine to form 3 is unfavorable. Interestingly, reducing 2 to 2 red leads to spontaneous proton transfer if a second-sphere histidine is doubly protonated. Yet, calculated reduction potentials show that 2 is the least favorable species to reduce. We therefore calculated vertical electron attachment energies along the reaction path of the proton transfer from the second-sphere histidine. As these change drastically along the path, we conclude that PCET is the most likely option. Release of an OOH -species from the resulting [Cu-OOH] + is possible and we observe proton abstractions from water in the process, forming H 2 O 2 and Cu(II).