Kinetic information has been obtained (pH 2-6, 1 = 0.10 M (NaCl)) for the redox reactions of six inorganic complexes with the single (type 1) blue Cu protein rusticyanin from Thiobacillus ferrooxidans. The protein is unusual in that it is normally functional at pH ~2, at which pH it has a reduction potential of 0.68 V. From the dependence of rate constants on pH in three cases protein acid dissociation pKa values have been determined. These are as follows: with [Mn(Cydta)(H2O)]- as oxidant for RCuI, 4.17 at 25 °C; with [Ru(NH3)4(phen)]2+ as reductant for RCuII, 4.06 at 15 °C; with [Co(dipic)2]- as oxidant for RCu1, 4.10 at 25 °C. The reduction potential of rusticyanin decreases to 0.62 V at pH 5.3. The behavior observed suggests that protonation is at neither the active site nor binding site(s) on the protein. No other pKa's have been detected in studies up to pH 6. When the ionic strength is adjusted with Na2SO4, rate constants (pH 3–6) for the [Ru(NH3)4(phen)]2+ reduction are ~50% of those obtained with NaCl.