The reduction of two diamino-l,3,5-triazines and nine 1,3,5-triazine herbicides (simazine and other chlorinated herbicides, simetryn and other methylthio herbicides) on mercury electrodes are examined. The Cl-derivatives were reduced into two steps, being the first of them the reductive cleavage of a Cl atom and being the triazine ring reduced in the second step. For the –SCH₃ derivatives a complex EC(EE) mechanism was found, yielding the same end product than the other group of herbicides.