A modified carbon-paste electrode (CPE) is prepared by incorporating thionine-nation supported on multi-walled carbon nanotube (MWCNT). The electrochemical behavior of dopamine (DA) and ascorbic acid (AA) on the surface of the modified electrode is investigated by cyclic voltammetry (CV). The results show that thionine effectively immobilized in the matrix of the paste by using an appropriate mixture of nafion/MWCNT under the ultrasonic condition. On the other hand, presence of nation enhances the stability of the thionine supported by MWCNT in the composite electrode and improves the reproducibility of the surface of the modified electrode under renewing process by polishing. The results of cyclic and differential pulse voltammetric investigations show that the modified electrode possesses an efficient electrocatalytic activity for the electrochemical oxidation of DA and AA and a peak potential separation nearly 379 mV is resulted for two compounds. The prepared modified electrode does not show any considerable response toward the electro-oxidation of sulfhydryl compounds, such as, cysteine, penicillamine and glutathione, revealing a good selectivity for voltammetric response to AA and DA in clinical and pharmaceutical preparations. The effective electrocatalytic property, excellent peak resolution and ability for masking the voltammetric responses of the other biologically reducing agents, make the modified electrode suitable for simultaneous and sensitive voltammetric detection of sub-micromolar amounts of AA and DA. (c) 2007 Elsevier Ltd. All rights reserved.