The utility of carbon paste electrodes (CPE) modified with cobalt(II) phthalocyanine for electrocatalytic oxidation of guanine and single strand DNA (ss-DNA) in aqueous solution is demonstrated. The modification of a CPE with this compound results in excellent amplification of the guanine oxidation response of ss-DNA. The electrochemical behavior of the modified electrode and the mechanism of the oxidation of guanine and ss-DNA were investigated using cyclic voltammetry and rotating disk electrode methods. The transfer coefficient (alpha) for electrocatalytic oxidation of guanine and the diffusion coefficient of this substance under the experimental conditions were investigated in this study. Detection limits of 85 and 280 ng mL(-1) were obtained for guanine and ss-DNA, respectively. (C) 2004 Elsevier B.V. All rights reserved.