The binding and subsequent reactivity of dioxygen (O-2) upon binding to copper ion centers is of fundamental interest in chemical and biological processes. We provide here a detailed account of the reaction of O-2 with dicopper(I) complexes, involving O-2-reversible binding, followed by the stoichiometric aromatic hydroxylation of the ligand. Thus, tricoordinated dicopper(I) complexes [Cu-2(R-XYL)](2+) (R = H, MeO, t-Bu, F, CN, NO2; 1a-f) possess dinucleating meta-substituted xylylene ligands with two chelating tridentate bis [2-(2-pyridyl)ethyl] amine (PY2) moieties and a 5-R substituent. Upon reaction with O-2, dioxygen adducts [Cu-2(R-XYL)(O-2)](2+) (2a,c-f) form reversibly, and these subsequently yield 2-xylylene-hydroxylated products [Cu-2(R-XYL-O-)(OH)](2+)(3a-f), which are phenoxo- and hydroxobridged copper(II) complexes. The products 3 have been characterized via the X-ray structure of the parent complex 3a, and by their UV-visible, infrared, and room-temperature magnetic properties. Incorporation of the O-atom from dioxygen into the phenolic products has been proven by isotopic labeling experiments, except in the case of 3f; where workup results in an exchange reaction causing loss of the oxygen label. In read-ions of O-2 With 1 in dichloromethane at room temperature, 10-25% yields of unhydroxylated complexes [Cu-2(R-XYL)(OH)](3+) (5) are obtained. A stopped-flow kinetics study of O-2 reactions of 1 in CH2Cl2 demonstrates that [Cu-2(R-XYL)(O-2)](2+) (2a,c-f) complexes form reversibly, proceeding via the reaction 1 + O-2 reversible arrow 2 (K-1 = k(1)/k(-1); this is followed by the irreversible reaction 2 - 3 (k(2)). Analysis of temperature-dependent data which is accompanied by spectrophotometric monitoring yields both kinetic and thermodynamic parameters for R = H, t-Bu, F, and NO2.