A sensitive voltammetric method is developed for the determination of diazinon, an organophosphorus pesticide, using a Nafion(R)-coated glassy carbon electrode as an enrichment film. The analysis of diazinon in aqueous solutions using sensitive instrumental methods has gained importance. Investigation of the electrochemical behavior of diazinon on a Nafion(R)-coated glassy carbon electrode reveals the presence of a specific oxidation peak of analytical significance. The experimental parameters, such as pH, film thickness, pre-concentration potential, preconcentration time, and square-wave voltammetric parameters were optimized. Using this method, a linear calibration curve for diazinon was obtained up to 5 muM range in pH 1 citrate buffer solution with a detection limit (SIN = 3) of 75 nM.