Three new layered metal phosphate–oxalates [(CH3)2NH2]2[M2(HPO4)2(H2PO4)2(C2O4)]·xH2O (M = Al, Ga, In) have been synthesized. [(CH3)2NH2]2[Al2(HPO4)2(H2PO4)2(C2O4)] displays the highest proton conductivity among metal phosphate–oxalates so far.