Poly(thionaphtheneindole) (pTNI) conducting polymer has previously been observed to undergo a large variation in the electrical conductivity as a function of relative humidity. In this paper the electrical characteristics of this material as a function of the adsorbed water are studied. Isothermal current-time and current-voltage plots can be understood in terms of an ionic charge transfer inside the polymer, controlled by a diffusion limited electrodic process. The charge transfer at the electrodes is probably due to proton reduction at the cathode and water oxidation at the anode.