We report here a detailed study concerning the electrochemical behavior of Os(4,4'-dimethyl, 2,2'-bipyridine)(2)(1,10-phenanthroline 5,6-dione) complex, adsorbed on spectrographic graphite, and about its electrocatalytic activity for NADH oxidation. Cyclic voltammetric measurements, performed in aqueous phosphate buffer solutions, at different scan rates and DH values, allowed us: (i) to relate the redox response of the o-quinone ligand (phendione) to that of the Os(II) central ion; (ii) to confirm that, in aqueous solutions, the phendione based redox process globally involves two electrons and two protons; (iii) to estimate the rate constant for the heterogeneous electron transfer corresponding to the phendione redox couple (k(s) approximate to 20.1 s(-1)). The second order rate constant for electrocatalytic oxidation of NADH (k(1,[NADH]=0) = 1.9 x 10(3) M-1 s(-1), at pH 6.1) as well as its pH dependence (from pH 5.5 to 8.1) were evaluated from RDE experiments, using both Koutecky-Levich and Lineweaver-Burk data interpretations.