Copper(I), copper(II), and thallium(III) hexafluoromolybdates(V), prepared by the oxidation of the metals in acetonitrile with molybdenum hexafluoride (A. Prescott, D.W.A.Sharp, and J.M. Winfield, J. Chem. Soc., Dalton Trans. , 1975, 963) have been investigated by cyclic voltametry. Half wave potentials, E 1 2 V vs. Ag p+/Ag were obtained using a evacuable cell equipped with anexternal Ag p+/Ag electrode, enabling strict anerobic conditions to be maintained. A number of reversible or quasi-reversible electron transfer processes have been observed, enabling comparison with synthetic work to be made. Results for Cu I and Cu II hexafluoromolybdates(V) are in accord with preparative experience. MoF 6. Mo VI/Mo VE 1 2 +1.600V, oxidises Cu metal to Cu II in MeCN, and Cu II is reduced by Cu O to Cu I , Cu lI/Cu IE 1 2 = +0.750 or +0.710V for Cu I and Cu II solutes respectively, Cu I/Cu OE 1 2 = −0.720V not reversible. A wave at E 1 2 = −0.350V is assigned to Mo V/Mo IV by analogy with Ag I hexafluoromolybdate (D.W.A. Sharp, unpublished work). E 1 2 data for I 2 in MeCN, I 2/I 3 - = 0.280, I 3 −/I − = -0.116V, suggest that reduction of MoF 6 − by I is not likely, in contrast to the situation in SO 2 (A.J. Edwards and R.D. Peacock, Chem. Ind. , 1960, 1441). Reduction of MoF 6 − by Cu o in MeCN should be feasible, but appears to be very slow. Pure Tλ III hexafluoromolybdate(V) is obtained from Tλ o and MoF 6 only when the mole ratio MoF 6:Tλ>5:1. Smaller ratios produce yellow solids in which Mo:Tλ is ca . 2:1. Tλ III is a stronger oxidising agent than Cu II in MeCN, as oxidation of Cu I by Tλ III is rapid and quantitative. However a reversible electron transfer wave assignable to Tλ III/Tλ I is not observed in the expected fange +1.600 to +0.710V possibly because of solute-electrode interactions.