The activation of dioxygen (O2) by Cu(I) complexes is an ubiquitous process in biology and industrial applications. In tyrosinase, a binuclear copper enzyme, a μ-η2:η2-peroxodicopper(II) species is generally accepted to be the active oxidant. Reported here is the characterization and reactivity of a stable μ-η2:η2-peroxodicopper(II) complex at −80 °C using a secondary diamine ligand, N,N‘-di-tert-butyl-ethylenediamine (DBED). The spectroscopic characteristics of this complex (UV−vis, resonance Raman) prove to be strongly dependent on the counteranion employed and not on the solvent, suggesting an intimate interaction of the counteranions with the Cu−O2 cores. This interaction is also supported by solution EXAFS data. This new complex exhibits hydroxylation reactivity by converting phenolates to catechols, proving to be a functional model of tyrosinase. Additional interest in this Cu/O2 species results from the use of Cu(I)−DBED as a polymerization catalyst of phenols to polyphenylene oxide (PPO) with O2 a...