Two new phenothiazine derivatives, 3,7-di(m-aminophenyl)-10-ethyl-phenothiazine (DAEP) and 3,7-di(m-hydroxyphenyl)-10-ethyl-phenothiazine (DHEP), were used to modify solid graphite electrodes by adsorption from DMSO solution. Cyclic voltammetry and rotating disc electrode measurements, performed under different experimental conditions (pH, potential scan rate, rotation speed and NADH concentration) showed that: (i) both compounds undergo a quasi-reversible one electron / one proton redox process (k(s) = 0.73 s(-1) for DAEP and 0.38 s(-1) for DHEP), occurring at a formal standard potential which is not affected by the nature of substituent at positions 3 and 7; (ii) the pK(a) values of the adsorbed mediators are slightly different (6.9 for DAEP and 7.8 for DHEP); (iii) the second order rate constant for NADH electrocatalytic oxidation is significantly larger for DAEP (k(obs,[NADH]=0) = 6014 M-1 s(-1)) than for DHEP (k(obs,[NADH]=0) = 134 M-1 s(-1)). Considering the mediators' structure, we made an attempt to explain the higher electrocatalytic efficiency of DAEP mediator. (C) 2011 Elsevier B.V. All rights reserved.