The electrical conductivity at intermediate temperature of 150-250 degrees C and the activation energy for conductivity of composite proton conductors, 2NH(4)PO(3)-(NH4)(2)Mn(PO3)(4) and 2NH(4)PO(3)-(NH4)(2)SiP4O13, were investigated as a function of water vapor pressure, P-H2O. The activation energy decreased linearly with the natural logarithm of P-H2O indicating that water is chemically adsorbed to the electrolytes. The decrease in activation energy is possibly caused by formation of hydrogen bonds between the adsorbed water and the electrolytes. In addition, the pre-exponential factor of Arrhenius equation, sigma(0), increased with P-H2O. This suggests that the adsorbed water may generate additional mobile protons for the composite electrolyte. Therefore, the enhancement in the electrical conductivity of a NH4PO3-based electrolyte in a water-vapor rich atmosphere originates possibly from the decrease in activation energy as well as the increase in mobile proton concentration. (C) 2007 Elsevier Ltd. All rights reserved.