A fast and simple electroanalytical procedure was applied for the determination of methylparathion in a solution extracted from a typical Brazilian soil using square wave voltammetry and a glassy carbon electrode. The effects of pH, scan rate and surface poisoning were studied in order to establish the optimum conditions for the electroanalysis of methylparathion. It was observed that the substances commonly presented in the soil solution modify the voltammograms, which improves the current values and displaces the peak potential to a less negative value. This was attributed to the more alkaline pH caused by dissolved organic matter, mineral colloids and other substances in the soil solution. The best response was obtained in neutral or in slightly acidic solutions. In such conditions, the limits of detection were 0.32 mg L À1 (1.21 Â 10 À6 mol L À1 ) in pure water and 0.36 mg L À1 (1.37 Â 10 À6 mol L À1 ) in the soil extracted solution.

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