A multiwall carbon nanotubes modified electrode was prepared using ferrocenedicarboxylic acid as a mediator to be used as ail amperometric sensor for the simultaneous determination of 6-thioguanine and folic acid. The electrochemical behaviors of the compounds at this modified electrode were studied using cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. The results indicated that the chemically modified electrode exhibited an efficient electrocatalytic activity towards the oxidation of 6-thioguanine and folic acid it pH 9.0. Using chronoamperometry, the catalytic reaction rate constant was calculated at 8.2 x 102 (mol L-1)(-1) s(-1). The catalytic peak current was linearly dependent on 6-thioguanine and folic acid concentrations in the range of 0.01-100 mu mol L-1 6-thioguanine and 4.6-152 mu mol L-1 folic acid. The detection limits for 6-thioguanine and folic acid were 0.0085 mu mol L-1 and 1.1 mu mol L-1, respectively. The RSD% for 10 replicates determination of 0.70 and 11.0 mu mol L-1 of 6-thioguanine were 1.3% and 1.1%, respectively, whereas for 15.0 and 30.0 mu mol L-1 folic acid, they were 0.9% and 1.1%, respectively. The modified electrode showed good sensitivity, selectivity, and stability. It Was Successfully applied for the determination of both 6-thioguanine and folic acid in real samples. (C) 2010 Elsevier B.V. All rights reserved.