The voltammetric behavior of thioglycolic acid (TGA) was studied at a carbon paste electrode modified with cobalt phthalocyanine (CoPc). The CoPc-modified electrode shows high electrocatalytic activity toward oxidation of TGA, lowering substantially the overpotential of anodic reaction. Results of the cyclic voltammetry show that TGA undergoes a two-step oxidation (each step with one electron) resulting the dimer of thiol. Enhancement of the rate of electron transfer results in a near-Nernstian behavior of modified electrode to the concentration of TGA and makes it as a suitable potentiometric sensor for the detection of this compound. This electrode shows a near-Nernstian response in a wide linear range of the concentration TGA (similar to4 orders of magnitude). The modified electrode was used successfully for the determination of TGA and its salts in hair-treatment products and also in culture media. The modified electrode exhibited a fast response time (< 10 s), very good stability, and had an extended lifetime. (C) 2003 Elsevier Ltd. All rights reserved.