A rapid, sensitive method for the determination of paraquat and diquat in aqueous media using square wave voltammetry (SWV) is discussed on the basis of the analyte behaviour in cyclic voltammetry. Cation-radicals formed upon the one-electron reduction of the analyte(s) adsorb on the mercury electrode surface with both flat and vertical orientations. With low concentrations only the flat oriented deposit occurs and the adsorption becomes progressively diffusion-controlled, especially at high scan rates. SWV experiments were based on this diffusion-controlled adsorption process. Effects on square wave frequency f, staircase step height Delta E(s), scanning direction, presence of surfactants and supporting electrolyte concentration were examined for the optimization of experimental conditions. The optimal parameters found were f = 100 Hz and Delta E(s) = 2 mV for scanning potentials from -0.85 to -0.25 V in the presence of 0.003% gelatin and with supporting electrolyte concentrations ranging from 0.02 to 0.05 M. Calibration curves were linear in the range 5 x 10(-8) to 1 x 10(-5) M with a detection limit of 1.5 x 10(-8) M. SWV is a sensitive technique, more rapid than all other described procedures for determining these pesticides, and allows analysis to be performed in the presence of an ionic matrix. The method was extended to the analysis of paraquat and diquat in potatoes after digestion in hot sulphuric acid, neutralization and solid phase extraction. The detection limit has been found to be 1 mu g g(-1) for both pesticides.