Proton conducting electrolyte materials operational in the intermediate temperature range of 200-400 degrees C are of special interest for applications in fuel cells and water electrolysers. Bismuth phosphates in forms of polycrystalline powders and amorphous glasses are synthesized and investigated by scanning electron microscopy, X-ray diffraction, FT-IR, thermogravimetric analysis and AC impedance. Under dry atmosphere the pure crystalline and amorphous phosphates exhibit an intrinsic conductivity of up to 10(-5) S cm(-1) at 250 degrees C. In the presence of atmospheric humidity the conductivity of both types of phosphates is significantly enhanced, reaching about 10(-2) S cm(-1) at a water vapor partial pressure above 0.5 atm. During a period of more than 100 h with four humidity cycles from zero to 0.58 atm of the water vapor partial pressure, the phosphates show good stability, suggesting the potential as an intermediate temperature electrolyte. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.