Two new compounds, [(C₄N₂H₁₂)₃][P₂Mo₅O₂₃]·H₂O,I, and [(C₃N₂H₁₂)₃][P₂Mo₅O₂₃]4H₂O,II, have been prepared employing hydrothermal methods in the presence of aliphatic organic amine molecules. Both the compounds possess the same polyoxoanion, pentamolybdatobisphosphate, (P₂Mo₅O₂₃)^6-. The anions consist of a ring of five MoO₆ distorted octahedra with four edge connections and one corner connection. The phosphate groups cap the pentamolybdate ring anion on either side. The anion is stabilized by strong hydrogen bonds involving the hydrogen atoms of the amine molecules and the oxygen atoms of the polyoxoanion and water molecules. Crystal data:I, monoclinic, space group = P2₁/n (no. 14), mol. wt. = 1192.1,a = 9.4180(1),b = 18.1972(3),c = 19.4509(1) å, Β = 103.722(1)‡,V = 3238.37(7) å³, Z = 4;II, triclinic, space group = P1 (no. 2), mol. wt. = 1210.1,a 9.5617(9),b = 13.3393(12),c = 13.7637(12) å, α = 88.735(1), Β = 75.68(1), γ = 87.484(2)‡,V = 1699.2(3) å³.