The mixed-valent 10-manganese(III/IV)-containing 36-tungsto-4-arsenate(V), [MnIII
6MnIV
4O4(OH)12(H2O)12(A-β-AsW9O34)4]22−
Interaction of the mixed-valent 12-manganese coordination complex [MnIII 8MnIV 4O12(CH3COO)16(H2O)4] with the lacunary 9-tungstoarsenate(V) [A-α-AsW9O34]9− resulted in the 10-manganese(III/IV)-containing 36-tungsto-4-arsenate(V), [MnIII 6MnIV 4O4(OH)12(H2O)12(A-β-AsW9O34)4]22− (1). Polyanion 1 was isolated as a hydrated mixed potassium–sodium salt, K14Na8[MnIII 6MnIV 4O4(OH)12(H2O)12(A-β-AsW9O34)4]·104H2O, which crystallizes in the orthorhombic space group Pbcn and was characterized by FT–IR spectroscopy and single-crystal X-ray diffraction, as well as elemental and thermogravimetric analyses. The title polyanion contains a unique [MnIII 6MnIV 4O4(OH)12(H2O)12]14+ core stabilized within the 36-tungsto-4-arsenate(V) framework.