Reduction potentials E o′ for the T. versutus amicyanin couple, AmCu II/I, were determined at pH values in the range 4.4–9.0 by direct measurement using cyclic voltammetry, and from rate constants for the reactions AmCu I + [Co(terpy) 2] 3+ and [Co(terpy) 2] 2+ + AmCu II, using an E o′ for the [Co(terpy) 2] 2+/3+ couple of 260mV. At pH > 7.5 the value obtained is 236 mV, which increases with decreasing pH in keeping with proton inactivation of AmCu I Together with previously determined E o′ values for the T. versutus cytochrome c 550 Fe III/Fe II couple, it is concluded that the physiologically relevant reaction AmCu I + cyt c 550Fe III ( k f) is thermodynamically favourable at pH > 6.25, but that the back reaction cyt c 550Fe II + AmCu II ( k b) is favourable at pH < 6.25. Values of k f (25°C) at pH > 6.25 were determined directly by the stopped-flow method, I = 0.100 M (NaCl). At pH < 6.25 k f values were obtained indirectly from the measured k b and equilibrium constants from ΔE o′ . The combined k f variations with pH give an acid dissociation p K a for AmCu IH + of 6.6. In further studies (25°C) rate constants/M −1s −1 (pH 6.0–8.6) were determined for the cross-reactions of AmCu I with P. aeruginosa azurin AzCu II, and AmCu I with P. aeruginosa cyt c 550Fe III , and are 11.0 × 10 5 and 6.4 × 10 5 M −1s −1 respectively at pH 8.6. Using the Marcus equations corresponding electron self-exchange rate constants ( k ese/M −1s −1) of 1.3 × 10 5 and 0.6 × 10 5 M −1s −1 were calculated for the exchange of AmCu II with unprotonated AmCu I in good agreement with the value 1.2 × 10 5 M −1s −1 determined by NMR at pH 8.6. Information was also obtained as to the effect of pH on these k ese values.