The reactivity towards dioxygen has been explored for two copper(I) tris(pyrazolyl)-methane complexes, [Cu I ((3,5-Me 2 -pz) 3 CH)(MeCN)](PF 6 ) 2 (1) and [Cu I ((3,5-Me 2 -4-Br-pz) 3 )(MeCN)](PF 6 ) 2 (2). Reversible oxygenation of 1 occurs in CH 2 Cl 2 solution at ~7 o C to yield a 2:1 μ-η 2 η 2 -peroxo complex [((3,5-Me 2 -pz) 3 CH) 2 Cu 2 (μ-O 2 )](PF 6 ) 2 . Equilibrium studies, made using manometric techniques over the temperature range 258-278 K and at two equilibration times, 2 and 30 min, yielded best-fit thermodynamic constants of K 2 1 =32 M - 2 (at 298 K), ΔH o =-110+/-5 K J mol - 1 , ΔS o =-340+/-18 J K - 1 mol - 1 (30 min) which have been compared to values for other CuO 2 adducts. Thermal decomposition of this μ-peroxo species yields crystals of a di-μ-hydroxo complex [((3,5-Me 2 -pz) 3 CH) 2 Cu 2 (μ-OH) 2 ](PF 6 ) 2 (3), whose crystal structure and magnetism (J=-298 cm - 1 ) are described. Complex 3 can also be made by reaction of 1 with iodosylbenzene. Complex 2 does not form a dioxygen adduct but slowly oxidizes in CH 2 Cl 2 to form [Cu I I ((3,5-Me 2 -4-Br-pz) 3 CH) 2 ](PF 6 ) 2 .2CH 2 Cl 2 (4), the crystal structure of which is described. Complex 4 was also formed by the reaction of 2 with iodosylbenzene.