Molybdenocene dichloride (CP2MoCl2,) has been reported to exhibit in vivo and in vitro antitumor activity, inhibiting DNA biosynthesis in cells. At the present time, its mechanism of action is poorly understood. Currently, there is a discrepancy in what has been reported about the interaction Of CP2MoCl2 and DNA at physiological pH. The discrepancy is whether or not CP2MoCl2 binds DNA at pH 7.4. Our electrochemical studies indicate that, in fact CP2MoCl2(aq) interacts with the nitrogen bases present in DNA (adenine, cytosine, guanine and thymine) at pH 7.4 in buffer solution, showing a higher affinity for guanine, followed by adenine and showing very poor affinity for pyrimidine bases. The strength of the interaction is identified by the induced change in the oxidation potential of Mo(IV)-base as compared to the free unbound molybdenocene species. In order to corroborate the effect of nucleobases on molybdenocene, we investigated the interaction Of Cp2MoCl2(aq) with the nitrogen bases using U-V-Vis spectroscopy, in buffer solutions at physiological pH. It was observed that all nitrogen bases interact with Cp2MoCl2(aq) with parallel behavior to that of cyclic voltammetry studies, that is an affinity in the order of guanine > adenine > cytosine > thymine. (C) 2003 Elsevier B.V. All rights reserved.