The protonation-reduction of a dioxygen adduct with [LCuI][B(C6F5)(4)], cupric superoxo complex [LCuII(O-2(center dot-))] (1) (L = TMG(3)tren (1,1,1-tris[2-[N-2-(1,1,3,3-tetramethylguanidino)]ethyl]amine)) has been investigated. Trifluoroacetic acid (HOAcF) reversibly associates with the superoxo ligand in ([LCuII(O-2(center dot-))](+)) in a 1:1 adduct [LCuII(O-2(center dot-))(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, K-eq = 1.2 x 10(5) M-1 (-130 degrees C) and Delta H degrees = -6.9(7) kcal/mol, Delta S degrees = -26(4) cal mol(-1) K-1). Vibrational (rR) data reveal a significant increase (29 cm(-1)) in v(o-o) (= 1149 cm(-1)) compared to that known for [LCuII(O-2(center dot-))](+) (1). Along with results obtained from XAS and DFT calculations, hydrogen bonding of HOAcF to a superoxo O-atom in 2 is established. Results from NMR spectroscopy of 2 at -120 degrees 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 (Me(10)Fc) or octamethylferrocene (Me(8)Fc), leading to the products H2O2, the corresponding ferrocenium salt, and [LCuII(OAcF)](+). Second-order rate constants for electron transfer (k(et)) were determined to be 1365 M-1 s(-1) (Me(10)Fc) and 225 M-1 s(-1) (Me(8)Fc) at -80 degrees C. The (bio)chemical relevance of the proton-triggered reduction of the metal-bound dioxygen-derived fragment is discussed.