A square wave voltammetry (SWV) method for the determination of trace amounts of paraquat at carbon paste electrode modified with natural phosphate (NP-CPE) is proposed. Paraquat was accumulated onto natural phosphate at open circuit potential from aqueous solution. The reduction peaks of paraquat were observed around -0.7 V and - 1.0 V (versus SCE). The response of paraquat at NP-CPE related to: the concentration of this herbicide, preconcentration time, natural phosphate loading and measuring solution pH, was investigated. It was observed that the increase of the cathodic peak currents using SWV, under the optimized condition, is linear with the increase of paraquat concentration in the range from 2.3 x 10(-8) mol L-1 to 300 x 10(-8) mol L-1. The detection limit (DL, 3S.D.) and quantification limit (QL, 10S.D.) for peak 1 were about 7.8 x 10(-1)0 mol L-1 and 2.59 x 10(-9) mol L-1, respectively, in pure electrolyte water with the relative standard deviation of 1.8% (n = 7). The proposed method was successfully applied to the determination of paraquat in real water samples with satisfactory results. (c) 2007 Elsevier B.V. All rights reserved.