The Brdicka reaction has been widely used for protein analysis, even though its mechanism was not completely elucidated. It is usually performed in a solution of ammonia buffer and Co(NH3)(6)Cl-3 in differential pulse (DP) mode at the mercury electrode. A new interpretation of the existing results is presented leading to the complete elucidation of the mechanism. The signal with E-p = - 1.35 V versus the reference electrode corresponds to the reduction of the complex RS2Co, while the only signal which corresponds to the reduction of the hydrogen at the mercury electrode is the most negative one at E-p = -1.48 V. Based on the mechanism elucidated, it clearly follows that the Brdicka reaction is a complex one. Therefore, the crucial physico-chemical parameters like the temperature, pH, c(NH4+), c(Co(NH3)(6)Cl-3), c(protein) should be optimized and, in an analytical protocol, kept strictly in order to obtain intercomparable results on metallothionein analysis.