The trivalent copper complexes of GlyGlyHis, CumIII(H-2GlyGlyHis) and CumIII(H-3GlyGlyHis), prepared via the oxidation of the corresponding Cu(II) complexes by azide radicals, decompose rapidly to yield CO2, GlyGlyNHCH(OH)CH2Im (Im = imidazole) and Cu(I) in aqueous solutions. The decomposition of CuIII(H-2GlyGlyHis) consists of two consecutive first-order reactions. The first reaction is attributed to a decarboxylation process, where CO2 and a complex with a CuIII-carbon σ-bond are formed. This reaction is followed by a heterolytic decomposition of the latter complex to Cu(I) and GlyGlyNHCH(OH)CH2Im. CuIII(H-2GlyGlyHis), which is less stable than CuIII(H-3GlyGlyHis), decarboxylates with a rate constant of 38 ± 5 s-1, whereas the rate of decarboxylation of CuIII(H-3GlyGlyHis) is about 2 orders of magnitude slower. The pKa of the trivalent complex was determined to be 8.6 ± 0.2. A detailed pressure-dependence study of all the mentioned reaction steps was undertaken. The reported volumes of activation support the above outlined assignments for the nature of the rate-determining steps.