The protonation-reduction of a dioxygen adduct with [LCu I][B(C6F5)4], cupric superoxo complex [LCuII(O2•-)]+ (1) (L = TMG3tren (1,1,1-tris[2-[N2-(1,1,3,3-tetramethylguanidino) ]ethyl]amine)) has been investigated. Trifluoroacetic acid (HOAcF) reversibly associates with the superoxo ligand in ([LCuII(O 2•-)]+) in a 1:1 adduct [LCu II(O2•-)(HOAcF)]+ (2), as characterized by UV-visible, resonance Raman (rR), nuclear magnetic resonance (NMR), and X-ray absorption (XAS) spectroscopies, along with density functional theory (DFT) calculations. Chemical studies reveal that for the binding of HOAcF with 1 to give 2, Keq = 1.2 × 105 M-1 (-130 C) and ΔHo = -6.9(7) kcal/mol, ΔSo = -26(4) cal mol-1 K-1). Vibrational (rR) data reveal a significant increase (29 cm-1) in vO-O (= 1149 cm-1) compared to that known for [LCu II(O2•-)]+ (1). Along with results obtained from XAS and DFT calculations, hydrogen bonding of HOAc F to a superoxo O-atom in 2 is established. Results from NMR spectroscopy of 2 at -120 C in 2-methyltetrahydrofuran are also consistent with 1/HOAcF = 1:1 formulation of 2 and with this complex possessing a triplet (S = 1) ground state electronic configuration, as previously determined for 1. The pre-equilibrium acid association to 1 is followed by outer-sphere electron-transfer reduction of 2 by decamethylferrocene (Me10Fc) or octamethylferrocene (Me8Fc), leading to the products H 2O2, the corresponding ferrocenium salt, and [LCu II(OAcF)]+. Second-order rate constants for electron transfer (ket) were determined to be 1365 M-1 s-1 (Me10Fc) and 225 M-1 s-1 (Me8Fc) at -80 C. The (bio)chemical relevance of the proton-triggered reduction of the metal-bound dioxygen-derived fragment is discussed.