The dicopper(l) complex [Cu-2(MeL66)](2+) (where MeL66 is the hexadentate ligand 3,5-bis-{bis(2-(1-methyl-1H-benzimidazol-2-yl)-ethyl]-amino}-methylbenze ne) reacts reversibly with dioxygen at low temperature to form a mu-peroxo adduct. Kinetic studies of O-2 binding carried out in acetone in the temperature range from -80 to -55 degrees C yielded the activation parameters Delta H-1(not equal) = 40.4 +/- 2.2 kJ mol(-1), Delta S-1(not equal) = -41.4 +/- 10.8 J K-1 mol(-1) and Delta H--1(not equal) = 72.5 +/- 2.4 kJ mol(-1), Delta S--1(not equal) = 46.7 +/- 11.1 J K-1 mol(-1) for the forward and reverse reaction, respectively, and the binding parameters of O-2 Delta H degrees = -32.2 +/- 2.2 U mol(-1) and Delta S degrees = -88.1 +/- 10.7 J K-1 mol(-1). The hydroxylation of a series of p-substituted phenolate salts by (Cu-2(MeL66)O-2](2+) studied in acetone at -55 degrees C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant rho = -1.84. The temperature dependence of the phenol hydroxylation was studied between -84 and -70 degrees C for a range of sodium p-cyanophenolate concentrations. The rate plots were hyperbolic and enabled to derive the activation parameters for the monophenolase reaction Delta H-ox(not equal) = 29.1 +/- 3.0 kJ mol(-1), Delta S-ox(not equal) = -115 +/- 15 J K-1 mol-1, and the binding parameters of the phenolate to the mu-peroxo species Delta H(b)degrees = -8.1 +/-1.2 kJ mol(-1) and Delta S(b)degrees = -8.9 +/- 6.2 J K-1 mol(-1). Thus, the complete set of kinetic and thermodynamic parameters for the two separate steps of O-2 binding and phenol hydroxylation have been obtained for [Cu-2(MeL66)](2+).