Two crystalline metal phosphates (metal = Be and Al) were prepared under ionothermal conditions using a multifunctional ionic liquid as a solvent, a structure-directing agent, and a phosphorus source. The beryllium phosphate has a three-dimensional structure with intersecting 24-membered ring (24 MR) channels. The aluminum phosphate has a two-dimensional structure containing 8 MR windows. It displays exceptional hydrothermal stability and shows a high proton conductivity on the order of 10(-3) S cm(-1) at 25 degrees C under high humidity conditions.