A new tridentate ligand, PYAN, is employed to investigate solvent influences for dioxygen reactivity with [Cu(PYAN)(MeCN)]B(C6F5)(4) (1). Stopped-flow kinetic studies confirm that the adducts [{Cu-II(PYAN)}(2)(O-2)][B(C6F5)(4)](2) (2(Peroxo)) and [{Cu-III(PYAN)}(2)(O)(2)][B(C6F5)(4)](2) (2(Oxo)) are in rapid equilibrium. Thermodynamic parameters for the equilibrium between 2(Peroxo) and 2(Oxo) are as follows: THF, DeltaHdegrees similar to -15.7 kJ/mol, DeltaSdegrees similar to -83 J/K(.)mol; acetone, DeltaHdegrees similar to -15.8 kJ/mol, DeltaSdegrees similar to -76 J/K(.)mol. UV-visible absorption and resonance Raman spectroscopic signatures demonstrate that the equilibrium is highly solvent dependent; the mixture is mostly 2(Peroxo) in CH2Cl2, but there are significantly increasing quantities of 2(Oxo) along the series methylene chloride --> diethyl ether --> acetone --> tetrahydrofuran (THF). Copper(II)-N-eq stretches (239, 243, 244, and 246 cm(-1) in CH2Cl2, Et2O, acetone, and THF, respectively) are identified for 2(Peroxo), but they are not seen in 2(Oxo), revealing for the first time direct evidence for solvent coordination in the more open 2(Peroxo) structure.