Electrochemical behavior of fenitrothion (FT) and bifenox (BF) was investigated using cyclic voltammetry on the glassy carbon electrode. Cathodic peaks for the reduction of an Ar-NO2 group in both fenitrothine (o,o-dimethyl o-(3-methyl-4-nitrophenyl) phosphorothioate) (FT) and bifenox (methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate) (BF) overlap strongly, and it is difficult to determine the compounds individually from their mixtures by these cathodic peaks. By an electrochemical irreversible reduction of Ar-NO2 in FT and BF it was transformed into a reversible redox couple (Ar-NHOH/Ar-NO) which can be used for simultaneous determination of fenitrothion (FT) and bifenox (BF) by square wave voltammetry (SWV). The experimental parameters, such as pH value, prepotential magnitude, time of the applied prepotential and pulse amplitude of SWs were optimized. To obtain better detection limit and sensitivity, the surface of the glassy carbon electrode was modified by a pretreated multi-walled carbon nanotube. Scanning electron microscopy (SEM) and square wave voltammetry (SWV) were used to characterize the performance and structure of the modified electrode. Under the optimized experimental conditions FT and BF give linear response over the range of 0.2-60.0 mu M. The detection limit for both FT and BF was found to be 0.08 mu M. The practical application of the modified electrode was demonstrated by measuring the concentration of FT and BF in river water samples. (C) 2016 Elsevier B.V. All rights reserved.