The factors that control the diverse reactivity of the μ-η(2):η(2)-peroxo dicopper(ii) oxy-intermediates in the coupled binuclear copper proteins remain elusive. Here, spectroscopic and computational methods reveal H-bonding interactions between active-site waters and the μ-η(2):η(2)-peroxide of oxy-tyrosinase, and define their effects on the Cu(II)(2)O(2) electronic structure and O(2) activation.