A two-dimensional layered inorganic-organic hybrid metal hydrogenophosphate (1) was treated with 0.1 M NaOH-ethanol solution, which resulted in a Na+-ion substitution product that exhibits excellent thermal and aqueous stability with 1, as well as much higher proton conductivity (sigma = 10(-2) S.cm(-1)) even at low temperature (283 K). This is because Na+ ions in aqueous solution make a more dense and extensive H-bonding network of water molecules, which enables protons to more easily transfer along the network.