The reduction mechanism of ioxynil (3,5-diiodo-4-hydroxy-benzonitrile), bromoxynil (3,5-dibromo-4hyd roxybenzonitrile) and chloroxynil (3,5-dichloro-4-hydroxybenzonitrile) was studied in dimethylsulfoxide by the electrochemical methods combined with FTIR spectroelectrochemical and GC/MS identification of the products. Cyclic voltammetry at different scan rates and at various concentrations of pesticides is consistent with the radical anion formation followed by the subsequent chemical reactions. The main decomposition pathway includes the cleavage of the halogen atom and the dimerization of the dehalogenated intermediate. The differences in the mechanism of ioxynil, bromoxynil and chloroxynil are caused by the mutual differences in the rates of the bond cleavage and the dimerization process. (C) 2008 Elsevier B.V. All rights reserved.