Three　new　group　IIIA　metal　phosphate-oxalate　(MPO)　compounds,　namely　[(CH3)2NH2]2[M2(HPO4)2(H2PO4)2(C2O4)]　(M　=　Al　(1),　Ga　(2))　and　[(CH3)2NH2]2[In2(HPO4)2(H2PO4)2(C2O4)]·H2O　(3),　have　been　synthesized.　Their　crystal　structures　feature　an　anionic　layer　with　the　sql　topology　net.　In　particular,　1　displays　a　proton　conductivity　(σ)　of　9.09　×　10−3　S　cm−1　at　85　°C　and　under　98%　relative　humidity,　which　is　the　highest　among　MPOs.　This　study　not　only　endows　the　main　group　metal-based　MPO　family　with　new　members,　but　also　contributes　to　further　understanding　of　the　structure-directing　roles　of　amines　and　provides　a　feasible　idea　for　improving　the　proton　conductivity　of　MPOs.　©　2024　The　Royal　Society　of　Chemistry.