Voltammetric behavior of isoniazid (INZ) at an overoxidized polypyrrole (OPPy) modified glassy carbon electrode was investigated. The OPPy-modified glassy carbon electrode exhibited catalytic activity toward the electro-oxidation of INZ, which appeared as a reduced overpotential in a wide operational pH range of 1-13. The overall number of electrons involved in the catalytic oxidation of INZ was found 4 electrons. The diffusion coefficient of 3.6 x 10(-6) cm(2) s(-1) for INZ was also estimated using chronoamperometry study. It has been shown that using the OPPy-modified electrode, INZ can be determined by amperometry method with limit of detection of 3.15 x 10(-6) M. The method was successfully applied for analysis of INZ in solid pharmaceutical formulations. The results were favorably compared to those obtained by the reported USP method. The results of the analysis suggest that the proposed method has promise for the routine determination of INZ in the products examined. (c) 2006 Elsevier B.V. All rights reserved.