Solution studies indicate that the Cu2L(OH)3+ complex, where L is a large octaaza cryptand, reacts with NH3, KSCN and NaN3 to form the binuclear tertiary complexes Cu2L(NH3)24+, Cu2L(SCN)3+ and Cu2L(N3)3+. The equilibrium constants show a special stabilisation of the complexes with ligands able to bridge the CuII centres, with a maximum stabilisation for azide. Upon addition of an excess of acid, all the complexes decompose with release of Cu2+ and the protonated ligands. The kinetics of the acid-promoted decomposition have been studied under pseudo-first order conditions of excess acid. The decomposition of the OH−, SCN− and NH3 complexes is first order with respect to H+, although with a clear tendency to saturation, and the values of the rate constants depend on the nature of the ancillary ligand. In contrast, Cu2L(N3)3+ decomposes with a second order dependence on the acid concentration. These results are interpreted in terms of modifications induced by the ancillary ligand in the kinetics of the acid-assisted dissociation of CuN(cryptate) bonds. Depending on the steric requirements of the ancillary ligand, the CuN(cryptate) bonds are strained at different degrees and the more distorted complexes decompose faster. In the case of azide, there is an optimum fit between the ancillary ligand and the Cu2L4+ site that causes a change in the rate-determining step from dissociation of the first CuN bond to the second.