Cyclic voltammetry and electrochemical impedance studies of Fe(tmphen) 3 2+ (where tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline)-immobilized Nafion®-modified glassy carbon electrode (GC/Nf/Fe(tmphen) 3 2+ ) are carried out in 0.1 M Na2SO4 solution. Nafion–Fe(tmphen) 3 2+ complex exhibits efficient electrocatalytic oxidation of isoniazid. The linear double reciprocal plot of current and concentration of isoniazid shows a Michaelis–Menten-type catalytic process. The catalytic oxidation currents are proportional to the concentration of isoniazid and show a wide linear calibration range for the quantitative determination of isoniazid. Detection limit and sensitivity are found to be 13 μM and 2.5 μA mM−1, respectively.