Indium doped tin pyrophosphates were prepared by three synthetic routes. A heterogeneous synthesis from metal oxides with excess phosphoric acid produces crystalline phosphate particles with a phosphorus rich amorphous phase along the grain boundaries. The amorphous phase prevents the agglomeration of particles, hydrolyzes in moist atmosphere as revealed by FT-IR and solid state NMR, and facilitates a high proton conductivity (above 2.5 × 10-2 Scm− 1) with high stability at above 120 °C under a water partial pressure of 0.15 atm. This phase can be removed by washing with water, resulting in a dramatic decrease in conductivity as well as significant agglomeration of the particles, as evident in TEM and from particle size distribution measurements. Homogeneous synthesis with soluble metal acetates or chlorides as precursors results in a single crystalline phase with a small particle size, but strongly agglomerated, and a low conductivity at 10− 7–10− 6 Scm− 1 level. Further impregnation of the agglomerates with phosphoric acid does not lead to formation of the phosphorus rich amorphous layers on the surface of the crystals. An intermediate conductivity of 10− 3 Scm− 1 was observed for the acid treated phosphates from the chloride synthesis but no improvement for the acid treated phosphates from the acetate synthesis was observed.