The potentiometric response of three different platinum gas diffusion electrodes deposited on H3PO4 doped polybenzimidazole (PBI) was investigated under humidified atmospheres that contained H-2 or mixtures of H-2 and O-2. Continuum modelling was used to analyse the response. It is shown that the non-Nernstian response under H-2-H2O-N-2 mixtures can be explained by a difference of water activity on both sides of the membrane. Under H-2-O-2-N-2 mixtures, the oxygen mass transport parameters have a strong effect on the electrode sensitivity. © 2004 Elsevier Ltd. All rights reserved.