The electrochemical reduction of lead and oxidation reaction of the resulting Pb-9(4-) Zintl ions have been studied in 0.1 and 1.0 M KI solutions in liquid ammonia at -70 degrees C. The reduction process is a complicated multistep reaction. Under certain conditions the oxidation of Pb-9(4-) occurs as a simple, electrochemically irreversible reaction for which the limiting current is diffusion controlled. The diffusion coefficient of Pb-9(4-) was determined from a combination of chronoamperometry measurements on a 25 mu m Pt ultramicroelectrode and chronocoulometry measurements on a 1 mm Pb disk electrode. The chronocoulometric method for the determination of Pb-9(4-) determination is presented and used to determine the actual Pb-9(4-) concentrations in the electrochemical cell. Side effects, which accompany the reduction and oxidation reactions, have also been studied and the ways to avoid them presented.