The observation and fast time-scale kinetic determination of a primary dioxygen-copper interaction have been studied. The ability to photorelease carbon monoxide from [Cu-I(tmpa)(CO)](+) in mixtures of CO and O-2 in tetrahydrofuran (THF) between 188 and 218 K results in the observable formation of a copper-superoxide species, [Cu-II(tmpa)(O-2(-))](+) lambda(max) = 425 nm. Via this "flash-and-trap" technique, temperature-dependent kinetic studies on the forward reaction between dioxygen and [Cu-I(tmpa)(thf)]+ afford activation parameters DeltaH(double dagger) = 7.62 kJ/mol and DeltaS(double dagger) = -45.1 J/mol K. The corresponding reverse reaction proceeds with DeltaH(double dagger) = 58.0 kJ/mol and AS' = 105 J/mol K. Overall thermodynamic parameters are DeltaHdegrees = -48.5 kJ/mol and DeltaSdegrees = -140 J/mol K The temperature-dependent data allowed us to determine the room-temperature second-order rate constant, k(O2) = 1.3 x 10(9) M-1 s(-1). Comparisons to copper and heme proteins and synthetic complexes are discussed.