Spectroscopic studies of a mu-1,1-hydroperoxo-bridged copper dimer are combined with SCF-X alpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu2OOB unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts : v(O-O) = 892 cm(-1) v(as)(Cu-O) = 506 cm(-1), v(s)(Cu-O) = 322 cm(-1), and v(O-H) = 3495 cm(-1). The 892 cm(-1) O-O stretch of the mu-1,1-hydroperoxo-bridged copper dimer is 89 cm(-1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-1) O-O stretch are used to assign an electronic absorption band at 25200 cm(-1) (epsilon = 6700 M-1 cm(-1)) to a hydroperoxide pi*-to-Cu charge transfer (CT) transition. This band is similar to 5000 cm(-1) higher in energy than the corresponding transition in the unprotonated complex. The pi*-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotanated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi*-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.