The electrochemical behaviours of five sulfonamides (sulfanilamide, sulfadiazine, sulfamerazine, sulfamonomethoxine, sulfamethoxazole) were investigated with overoxidized polypyrrole (OPPy) modified pencil graphite electrodes. The performance of the OPPy electrode was evaluated by differential pulse voltammetry in Britton-Robinson buffer solutions prepared in different ratio of acetonitrile-water binary mixture, between pH 1.5 and 7.0. The highest anodic signals of sulfonamides were obtained in Britton-Robinson buffer solution prepared in 50% (v/v) acetonitrile-water at pH 2.5 and 3.0. The OPPy electrodes exhibited good performance for sulfonamides with wide linear ranges (approximate to 10(-5)-10(-3) M), highly reproducible responses (RSD% <= 0.92) and correlation coefficients (>= 0.9990). The calculated limits of detection were similar to 10(-6) or 10(-7) M at 3 sigma. In order to verify the reliability of the OPPy electrode as a sensor, it is used for determination of sulfamethoxazole in a pharmaceutical tablet. The recovery was found as 95.96% with the RSD% of 0.68. The overoxidized polypyrrole modified pencil graphite electrode showed a stable and reproducible response without any influence of interferent commonly existing in pharmaceutical containing sulfamethoxazole.