The electro-oxidation of glucose on nickel-modified glassy carbon electrode (GC/Ni) in a I M NaOH solution at different concentration of glucose was studied by the method of ac-impedance spectroscopy. In low concentration of glucose (<7 mM) two semicircles in the first quadrant of a Nyquist diagram were observed corresponding to charge transfer resistance and adsorption of intermediates. In higher concentration of glucose (>10 mM) negative resistance is observed in impedance plots as signified by semicircles terminating in the third quadrant. The impedance data in high concentration of glucose show different behavior at different applied anodic potential. The influence of the electrode potential on impedance pattern in high concentration of glucose is studied and a mathematical model was put forward to quantitative account for the impedance behavior of glucose oxidation. At potentials higher than 0.525V/Ag-AgCl, a pseudoinductive behavior is observed but at higher than 0.4 V/Ag-AgCl, impedance patterns terminate in the second and third quadrants. The conditions required for this behavior are delineated with the use of the impedance model. (C) 2008 Elsevier Ltd. All rights reserved.