The mu-1,2-peroxo dicopper(II) complex (2) of a compartmental bis(tetradentate) pyrazolate-based ligand is shown to convert, upon protonation, to the corresponding mu-1,1-hydroperoxo dicopper(II) complex (3). The transformation is cleanly reversed with base, and an apparent pK(a) = 22.2 +/- 0.3 for the Cu2OOH unit in MeCN has been determined. The unprecedented stability of 3 (t(1/2) = 9 h in nitrile solvents at room temperature, giving the hydroxo-bridged dicopper complex) has allowed for its structural characterization by X-ray diffraction. While the O-O bond length (1.462(3) angstrom) barely changes upon protonation from 2 to 3, the O-O stretching frequency is much higher in the hydroperoxo complex 3 (860 cm(-1)). 3 mediates 2e oxo transfer to the nucleophilic substrate PPh3 but is not activated for H-atom abstraction.