A tungsten wire covered with Na0.75WO3 acts in potentiometry as a reversible pH electrode having a pH dependent open-circuit potential E-ocp with nernstian slope. The mid-peak potential E-mp of cyclic voltammograms also depends on pH. At low pH (e.g., pH 2) and slow scan rates (e.g., 2 mV s(-1)) the voltammetric response is almost completely reversible. At higher pH and faster scan rates, the voltammetric systems exhibit features of increasing irreversibility. Under the conditions of reversibility, the E-ocp and E-mp differ significantly. E-ocp is determined by the proton transfer at the electrode surface; whereas E-mp is determined by the electron transfer equilibrium tungsten(VI)/tungsten(V) and the proton transfer at the electrode surface. The difference between E-ocp and E-mp provides the individual thermodynamic contributions of electron and proton transfer to the overall pH dependent redox electrode. This is the first time that both contributions can be separated for an insertion electrochemical system (thin surface layer). It is also shown for the first time that the mechanism of an ion-sensitive electrode can differ in potentiometry and voltammetry. (C) 2015 Elsevier B.V. All rights reserved.