The complexation of Cd with cysteine has been investigated at pH ~9, and in constant ionic medium 0.59 M NaNO₃ and borate buffer. Reduction signals of the hydrated Cd²⁺ and Cd-cysteinate complex were resolved on the potential scale during amperometric titration of 2 × 10^-6 M Cd²⁺ with cysteine and cysteine with Cd²⁺, in a differential pulse (DP) mode on HMDE. The 1:1 molar ratio of Cd²⁺ to cysteine, and cysteine to Cd²⁺, was defined for Cd-cysteinate complex formed under the defined conditions. The appearance of the prepeak on the Cd-cysteinate reduction signal is showed, as the result of cysteine adsorption in reduced form. From the mass balance equations, the stoichiometric stability constants of Cd-cysteinate complex were calculated. Under various experimental conditions, comparable log K values were obtained. The grand average log K = 7.83 M^-1 refers to 0.59 M ionic strength and 23℃.