The anionic [Cu-III(LH-4)](-) complex (L=8,17-dioxa-1,2,5,6,10,11,14,15-octaaza-tricyclo[13.3.1] eicosane-3,4.12,13-tetrone) is one of the rare copper(III) compounds, which exhibits high stability in basic aqueous solution. This copper(III) compound catalyses the reduction of molecular oxygen by L-ascorbic acid at pH 7.97. in Tris-HCl buffer solution at the ionic strength of 0.1 M (NaCl). Stoichiometry of the reaction (2:1.07 for L-ascorbic acid:dioxygen) indicates formation of water and dehydroascorbic acid as primary products. Based on detailed kinetic measurements, the rate equation-d[AscH(-)]/dt = k(obs)[AscH(-)](0.5) [Cu-III(LH-4)[O-2](0.5) was obtained. The proposed mechanism includes a fast redox pre-equilibrium between the copper(III) centre and its reduced, copper(II) form, induced by the presence of L-ascorbate. The equilibrium constant K-1' at pH 8(8.78 +/- 3.11 x 10(-3)) and k values for the forward and backward reactions (1.18 +/- 0.68 x 10(6) and 1.46 +/- 0.82 x 10(8) M-2 s(-1), respectively) were determined by stopped-flow technique, following the decrease in absorbance of the copper(III) form at 550 nm. In the presence of molecular oxygen, re-oxidation of the copper(II) form of the catalyst takes place, based on cyclic voltammetry (CV) measurements. The decrease of the Cu(II)-Cu(III) oxidation and the subsequent increase of the Cu(III)- Cu(II) reduction current peaks in the CV spectrum, when argon is exchanged to dioxygen atmosphere, indicate a relatively fast oxidation rate for [Cu-II(LH-4](2-). The determined Delta S double dagger 1 (-41 +/- 2 Jmol(-1) K-1) for the catalytic reaction indicate an associative mechanism for the formation of the catalytically active copper-oxygen species that will react with the L-ascorbate to yield dehydroascorbate as product. (C) 2010 Elsevier B.V. All rights reserved.