New low-energy intense bands are observed in the electronic absorption spectra of binuclear bridged transition-metal dimers which are not present in the spectra of the corresponding mononuclear compounds. These features are assigned as singlet charge-transfer transitions greatly lowered in energy due to large charge-transfer excited-state antiferromagnetic coupling. In addition, monomer charge-transfer transitions can split in energy in a dimer, and this splitting is experimentally observed to be much larger than that given by the interaction between charge-transfer transitions to the two coppers of the dimer. A valence bond configuration interaction model is presented which is applied to the peroxide --> Cu(II) charge-transfer spectrum of oxyhemocyanin and to the charge-transfer spectrum of a binuclear copper peroxide model complex with the side-on bridging (mu-n(2):n(2)) structure. This model accounts for both the excited-state antiferromagnetism and the energy splitting of the peroxide --> Cu charge-transfer transitions in the dimer. The observed charge-transfer excited-state spectral features provide insight into the bonding of the bridging ligand to the two metal centers. This bridging ligand is responsible for ground-state antiferromagnetic coupling between the metal centers, and thus the charge-transfer transitions of the bridging ligand provide a direct probe of the superexchange pathway in bridged metal dimers.