Two heterometallic decavanadate(V) compounds, bis(tetramethylammonium) decaaquadi-μ₄-oxido-tetra-μ₃-oxido-hexadeca-μ₂-oxido-hexaoxidodimanganese(II)decavanadate(V) pentahydrate, (Me₄N)₂[V₁₀O₂₈{Mn(H₂O)₅}₂]·5H₂O,A, and bis{[tris(hydroxymethyl)methyl]ammonium} decaaquadi-μ₄-oxido-tetra-μ₃-oxido-hexadeca-μ₂-oxido-hexaoxidodimanganese(II)decavanadate(V) dihydrate, [NH₃C(CH₂OH)₃]₂[V₁₀O₂₈{Mn(H₂O)₅}₂]·2H₂O,B, have been synthesized under mild reaction conditions in an aqueous medium. Both polyanions present two [Mn(OH₂)₅]²⁺complex units bound to the decavanadate cluster through oxide bridges. InA, the decavanadate unit has 2/msymmetry, whereas inBit has twofold symmetry. Apart from this, the main differences betweenAandBrest on the organic cations, tetramethylammonium and [tris(hydroxymethyl)methyl]ammonium, respectively, and on the number and arrangement of the water molecules of crystallization. In both compounds, the H atoms from the coordinating water molecules participate in extensive three-dimensional hydrogen-bonding networks, which link the cluster units both directly and through solvent molecules and, inB, through the `tris' cation hydroxyl groups. The cation inBalso participates in N—H...O hydrogen bonds. A number of C—H...O interactions are also observed in both structures.