The protonic conduction of H-mordenite has been investigated by various measurements. A hydrogen concentration cell using mordenite as a solid electrolyte gave rise to a stable EMF, in which the potential of the electrode with high partial pressure of hydrogen was negative. In addition, a stable and steady current could be drawn from a hydrogen-air fuel cell. The proton conductivity of mordenite at RT was 1.07 X 10(-5) S cm(-1). As the temperature increased, the conductivity once decreased, but then increased according to Arrhenius behavior. TGA measurement suggested that the decrease in conductivity with temperature was due to a decrease in amount of the adsorbed water. From the compositional dependence of the conductivity and the H/D isotope effect on the conductivity, two conduction mechanisms were proposed.