In electrochemical cells based on Nafion (R), exposed to a humid atmosphere that contains oxygen, hydrogen or a mixture of oxygen and hydrogen, the open circuit voltage, V-oc, depends linearly on the logarithm of the concentrations [H-2] and [O-2] in the gas phase. We show, experimentally, that the slope of these relations is significantly higher than in a concentration cell based on a free proton conductor. This was explained before to be due to a mixed potential formed by the oxidation of hydrogen. However, this explanation cannot be applied when one of the reacting gases H-2 or O-2 is missing. We suggest a different explanation that involves reactions that are in equilibrium and others totally blocked. With this mixed equilibrium-non-equilibrium approach (MENEA), we are able to interpret all the experimental results obtained under different conditions. in the analysis of these experiments, we always find a water transference coefficient m similar to 3, as expected for the species H+. (H2O)(m) that transports protons in highly humidified Nafion (R). The explanations are consistent with (a) Ag and Pd catalyzing the absorption of water by Nafion (R), (b) Pt and Pd catalyzing the reaction of oxygen and hydrogen with charged complexes in Nafion (R), and (c) in the absence of oxygen, Ag, Pt and Pd forming an oxide with water.