Phenothiazine derivatives containing two linearly condensed phenothiazine moieties, 16H,18H-dibenzo[c,1]-7,9-dithia-16,18-diazapentacene (I), dibenzo[e,1]-16,18-diacetyl-7,9-dithia-16,18-diazapentacene (II) dibenzo[c,1]-16,18-dibenzoyl-7,9-dithia-16,18-diazapentacene (III), and 16H,18H-dibenzo[c,1]-7.9-dithia-16,1 8-diazapentacene-7,7,9,9-bis-dioxide (IV), strongly adsorb on spectrographic graphite resulting in modified electrodes with electrocatalytic activity for NADH oxidation. From cyclic voltammetry measurements, performed in aqueous buffer solutions at different potential scan rates and pH values, the rate constants of the heterogeneous electron transfer and the transfer coefficients were estimated. The linear dependence between the peak current and the potential scan rate, corroborated with the slope of the formal standard potential versus pH linear regression, pointed out to a quasi-reversible, surface confined redox process involving le(-)/1H(+). The electrocatalytical efficiency, evaluated from cyclic voltammetry, and the second order electrocatalytical rate constant (k(1)), calculated from rotating disk electrode experiments, revealed the same sequence of the activity decrease: III > II > IV > I. Compound III-modified electrodes were characterized by the highest k(1) (1.8 x 10(3) M-1 s(-1), at pH 7.0) as well as by the highest stability, expressed by the lowest rate of the surface coverage depleting under continuous potential cycling.