The free metal ion potential, E(M), is a critical parameter in the calculation of formation constants when using voltammetry. When studying complex formation of Bi(III), however, E(Bi) cannot be directly measured. In this work a nitrate background electrolyte was employed to obtain reversible reduction waves. To determine E(Bi), measurements have to be made below pH ∼ 2 before the bismuth-oxy-nitrate species precipitates and thus corrections for the diffusion junction potential (monitored using Tl(I) as an internal reference ion) must be made. Additionally shifts in potential due to both Bi(III) hydrolysis and Bi(III) nitrate formation must also be compensated for before E(Bi) can be evaluated. The value of E(Bi) was determined relative to E(Tl) so that in an experiments where ligand is added to determine formation constants, E(Bi) can be determined as accurately as possible (since E(Tl) can generally still be measured). The value of E(Bi) - E(Tl) was found to be 495.6 ± 1.4 mV for the conditions employed.