A high proton-conducting phase appears in the composites of zirconium- and titanium-oxide nanoparticles and polyphosphoric acid (HPO3). Metal oxide nanoparticles (ZrO2 and TiO2) react with HPO3 and form composite electrolytes containing pyrophosphates (ZrP2O7 or TiP2O7) and shortened HPO3 chains. The ZrO2-HPO3 composite exhibits eleven times higher conductivity than sole HPO3 at the maximum. A formed layer of shortened HPO3 chains surrounding the pyrophosphates enhances the proton conductivities of the composite electrolytes and reduces the activation energies for the proton conductivities from 50 to 30 kJ mol(-1).