Solutions of copper(II) and copper(I) sulphate in water containing acetonitrile have applications in the hydrometallurgy of copper. The potentials (n.h.e.) of the Cu+/Cu; the Pt/Cu2+, Cu+, and the Cu2+/Cu electrodes in water containing a large molar excess of various copper(I) bases and various counter-anions have been measured. Free energies of transfer of copper(I) ions from water to water containing the bases are calculated. ΔGtr(Cu+) at 25�C becomes less exoenergetic for water containing the bases in the following order: CN- >> S2O32- > Me2NCHS > KI > NH3 > C5H5N > KBr > KCl > C3H5OH ≥ MeCN, CO, C2H4 > C3H6. The reduction potentials of CuSO4/Cu2SO4 in acidic saturated copper(II) sulphate solutions and a range of Cu2SO4 concentrations in acetonitrile- water mixtures show that CuSO4/MeCN/H2O is a powerful oxidant, comparable in strength to acidic iron(III) sulphate in water. Acetonitrile is the preferred base for the processing of copper by way of solutions of copper(I) sulphate.