A multi wall carbon nanotubes (MWCNT) modified glassy carbon electrode (GCE) was prepared by casting MWCNT suspension on the surface of the GCE. Evaporation of solvent from such a solution yielded robust surface film containing MWCNT. The electrochemical reaction mechanism of Sudan I was investigated at the MWCNT modified GCE (MWCNT/GCE) in phosphate buffer solution (PBS) with various pH values. Cyclic voltammetry, controlled-potential electrolysis studies showed the reduction of -N=N-group in Sudan I yielded aniline and alpha-amino naphthol through hydrazo (an intermediate product). The process was proved to be the ECE mechanism. In addition, the MWCNT/GCE can greatly catalyze the oxidation of -OH group in Sudan I with minimal electrode surface fouling. Consequently, an amperometric sensor for Sudan I was proposed. The sensor has excellent performance associated with high sensitivity (21.56 mu A mM(-1)), low detection limit (3.46 x 10(-8) M) and wide linear range (1.01 x 10(-6)-1.22 x 10(-4) M) in addition to stable response. Finally, the method was successfully applied for the determination of Sudan I in ketchup samples with satisfying results. (C) 2009 Elsevier B.V. All rights reserved.