The dinuclear copper(I) complex of 1,3-bis[N-(2-pyridylethyl)formimidoyl]benzene, [Cu-2(H-BPB-H)(CH3CN)(2)](BF4)(2), as well as the 5-nitro derivative, [Cu-2(NO2-BPB-H)(CH3CN)(2)](BF4)(2), react with dioxygen to form phenolate-bridged complexes as products. In a detailed kinetic study activation parameters of Delta H double dagger = 47 +/- 9 kJ/mol, Delta S double dagger = -53 +/- 11 J/(mol K), and Delta V double dagger = -9.5 +/- 0.5 cm(3)/mol for the reaction of [Cu-2(H-BPB-H)(CH3CN)(2)](BF4)(2) with dioxygen were obtained which account, together with further kinetic findings, for the occurrence of an intermediate peroxo complex that cannot be observed spectroscopically. The crystal structures of the products of the reaction were determined. Crystal data : complex [Cu-2(H-BPB-O)-OH(H2O)](BF4)(2), formula C22H24B2-Cu2F8N4O3, monoclinic space group P2(1)/c, Z = 4, a = 10.122(2) Angstrom, b = 28.711(6) Angstrom, c = 9.283(2) Angstrom, alpha = 90 degrees, beta = 100.78(3)degrees, and gamma = 90 degrees; complex [Cu-2(NO2-BPB-O)-OH(H2O)(3)](BF4)(2), formula C22H27B2Cu2F8N5O7, triclinic space group, , Z = 2, a = 10.144(2) Angstrom, b = 10.7612(2) Angstrom, and c = 16.000(4) Angstrom.