The sensing behavior of a potentiometric hydrogen sensor based on polybenzimidazole doped with phosphoric acid as the electrolyte and an E-TEK gas diffusion electrode as the sensitive electrode at room temperature is reported. The open-circuit potential (OCP) in N-2-N-2 mixtures obeys the Nernst equation with an accuracy equal to 0.1% of the hydrogen partial pressure. The 90% response time is equal to 10 s. In the presence of water vapor, an additional junction potential must be taken into account to describe the potentiometric behavior. The OCP in air containing hydrogen shows a sharp transition located at a H-2/O-2 ratio equal to 0.61. For hydrogen partial pressures lower than 8 X 10(3) Pa, it varies linearly with the logarithm of the hydrogen partial pressure with a slope almost equal to 130-140 mV/dec which exceeds the Nernst's value. The shape of the potentiometric response under open-circuit conditions is explained on the basis of the mixed potential model. The 90% response titne in air is equal to a few minutes. The presence of water vapor slightly influences the potentiometric response.