The voltammetric behaviour of the M(III/II) redox couple of the complexes of the general formula [M(diars)(2)X-2](+) where M = Os, Re, Ru, Rh and diars = o-phenylenebis-(dimethylarsine) has been investigated in aqueous and aqueous/ethanol solvents at bare and Nafion modified electrodes. The formal reduction potential falls within the range of 0.097-0.0920 V and increases with the dielectric constant of the solvent. In acertonitrile, both the oxidized and reduced forms of the complex showed no adsorption at the surface of the glassy carbon electrode as shown by cyclic voltammetry. The electrochemistry is consistent with a quasireversible one electron transfer between the di and tri cationic forms of the complexes. A current-enhancement of similar to 2 was observed at Nafion-modified glassy carbon over the bare electrode. Well defined voltammograms were obtained at Nafion-modified glassy carbon with a detectable signal down to 10(-9) M range by differential pulse voltammetry. Electrochemical detection of dopamine in presence of [Os(4-mediars)(2)Cl-2](+) at Nafion modified electrode indicates an increase in peak current. This shows that this complex acts as biochemical-sensor for dopamine.