A detailed study of the Cd(II)/Cd(Hg) system in 2-mercaptoacetic acid, H(2)MA, solutions has been carried out in 1 M NaClO4 at pH 4.8-12.0. The electrochemical behavior of 2-mercaptoacetic acid at a dropping mercury electrode was studied as well. Since all complexes that are formed in the solution undergo a reversible two-electron reduction, mathematical analysis of the data, provided the composition of the various complexes. Thus, the stability constants of the following complexes : Cd(SCH2CO2)(3)(4-), Cd(SCH2CO2)(2)(2-), Cd(SCH2CO2), CdH(SCH2CO2)(3)(3-), CdH2(SCH2CO2)(3)(2-), CdH(SCH2CO2)(2)(-), CdH2(SCH2CO2)(2) and CdH(SCH2CO2)(+) were determined and their degree of formation as a function of pH were estimated. In addition, the protonation constants of the ligand were calculated. The present results indicate that H(2)MA forms extremely stable bidentate complexes with cadmium ions. The formation of the various complexes as a Function of pH reveals that at pH 10.7-12.0 the complex Cd(SCH2CO2)(3)(4-) predominates, whereas at pH 4.8-6.0 the complexes CdH2(SCH2CO2)(2) and CdH(SCH2CO2)(+) are dominant. At pH 6.1-10.6 different protonated and unprotonated complexes composed of one to three ligands, coexist.