On the basis of the Nernstian pH-potential response, E=E 0 '-2.303(RT/F)pH, according to the reaction Pt+H 2 O->PtOH+H + +e - at the platinum surface, in this work the electric potentials of platinum wires treated by radio-frequency plasmas of oxygen, hydrogen and helium were measured. The potential was related theoretically to the surface coverage (θ) of the oxide layer at the platinum surface by means of the equation: E 0 '=E 0 + 2.303(RT/F)log[θ/(1-θ)]. By using this relationship, the effect of the plasma treatments was successfully explained: (1) an oxygen plasma oxidized the platinum surface to enlarge the oxide layer, (2) a hydrogen plasma reduced the oxide layer to diminish it and (3) the helium plasma, since it has no reducing power, might etch the oxide layer to remove it from the surface.