The cyclic voltammetric behaviour of cystine (RSSR) was studied on a static mercury drop electrode (SMDE) in the hanging mercury drop elecrode (HMDE) mode at pH 7.4. RSSR gave two peaks at -0.106 V (mercuric cysteine thiolate, RS-Hg-SR) and -0.552 V (mercurous cysteine thiolate, RS-HgHg-SR), respectively. The electrochemical behaviour of complexes of RSSR or folic acid with cadmium(II), and cadmium(II)-cysteinate (Cd(SR)2) complex in the presence of folic acid was also investigated. Experimental results show that the Cd(SR)2 complex forms a peak at -0.808 V on the mercury electrode surface. Also, the electroreduction of the cadmium(II)-folic acid complex occurs at -0.722 V. These complexes are reduced at a more negative potential than that of the free cadmium ion. When folic acid is added to RSSR solution, it causes shifts of the voltammetric reduction peaks of RS-Hg-SR and RS-HgHg- SR towards positive potentials. The shifts are invariably accompanied by a decrease of peak currents. This is explained as due to the formation of adducts between folic acid and cysteine thiolate (reduction product RS-) at the electrode surface. Analogous results were observed between folic acid and Cd(SR)2 complex, and were ascribed to the same cause.