This paper presents polarographic [direct current and differential pulse (DP)] studies of the electroreduction of the s-triazine derivatives prometryne [2-methylthio-4,6-di(isopropylamino)-1,3,5-triazine], desmetryne [2-methylthio-4-metliylamino-6-isopiopylamino-1,3,5-triazine], and terbutryne [(2-methylthio-4-ethylamino-6-terbutylamino)-1, 3,5-triazine] on mercury electrodes. The studies are performed in the acidity range 2.25 MH2SO4 to pH 6.5. Above this last pH value, no signals were obtained. fit DP polarography, two main reduction peaks were observed, accompanied by a prepeak, at less negative potentials, due to the adsorption of the herbicides on the electrode. The main peaks corresponded to two-electron irreversible reduction processes, at pH values higher than the protonation pK of the triazine ring (ca. 4). In this pH range, the protonation of the triazine ring precedes the reduction process, this reaction being also responsible for the observed decrease in limiting current. In strongly acidic media (pH < pK) two alternative reaction pathways are considered and the corresponding theoretical expressions are presented. The main conclusion in this pH range is that the herbicides suffer a cleavage of the -SCH3 group via two different intermediates related by a chemical reaction, whose importance depends on the herbicide.