The reduction of Pb(II) in the aqueous solution with some amino acids (cystein and serine) and thiodiglycolic acid (TDGA) have been investigated at dropping mercury electrode (DME). The stability constants of Pb-TDGA and Pb-amino acids were measured first by the method of Deford and Humes and that of Pb-amino acids-TDGA have been studied by the method of Shaap and McMasters. The reduction of all the complexes has been found to be reversible and diffusion controlled, involving two electrons in each case. Three types of mixed complexes MXY, MX₂Yand MXY₂ have been determined and found to be more stable than that of the simple ones. With the help of these data statistical and electrostatic effects have been considered.