A systematic study of composite powders of niobium oxide phosphate (NbOPO4) and phosphoric acid (H3PO4) has been performed in order to characterize the material's ability to perform as an electrolyte material in medium temperature fuel cells and electrolyzers. Powders of H3PO4 contents between 13.1 and 74.2 M% were produced and characterized with powder X-ray diffraction, P-31 MAS NMR and impedance spectroscopy. NMR revealed that a significant degree of dehydration and vaporization of H3PO4 takes place above 200 degrees C, and increases with temperature. At 500 degrees C the NbOPO4 and H3PO4 has reacted to form niobium pyrophosphate (Nb2P4O15). Impedance spectroscopy showed an increase in conductivity with increasing acid concentration, whereas the conductivity decreased slightly with increasing temperature. The highest conductivity measured was 2.5.10(-3) S/cm for a sample containing 74.2 M% of H3PO4. Lastly, it was shown that NbOPO4 has no significant conductivity of its own. (C) 2014 Published by Elsevier Inc.