The reduction mechanism of Cu(II) complexes of histidine and histidyl peptides was investigated using the pulse radiolysis method. The hydrated electron, used as a reducing agent, was found to react with the complexes by a bimolecular process. In the Cu(II)-histidine and Cu(II)-glycyl-histidine complexes at pH 11, the reaction with the e(aq)- is direct with the Cu(II) ion and no secondary reactions were found to occur up to 5 msec after the pulse. In the Cu(II)-glycyl-histidine complex at pH 6.5 and Cu(II)-β-alanyl-histidine at pH 7.5, the decay of the electron was followed by the appearance of a transient absorption band centered at 360 nm. This band is assigned to the adduct of an electron in the imidazole ring of the histidyl peptides. The radical ion is formed by the direct reaction of the e(aq)- with the liganded histidyl residue. The Im- decays via a first order process at the same rate at which the Cu(II) ion is reduced. CO2- reduces all the complexes studied via a bimolecular reaction. It is suggested that this is a direct reaction of the CO2- with the metal ion. An attempt to correlate these reaction steps to the reactivity of the reductants and to the structure of the complexes is made.