The reaction of [Mn12(CH3COO)16(H2O)4O12] · 2CH3COOH · 4H2O, Na8[HPW9O34] · 24H2O, K2HPO4, and ethylenediamine hydrochloride in an aqueous solution leads to the isolation of a new polyoxometalate-based mixed-valent manganese aggregate, K14Na17[(MnIII13MnIIO12(PO4)4(PW9O34)4)· ~56H2O (1). Single-crystal X-ray diffraction indicates that the polyoxoanion of 1 exhibits a mixed-valent [MnIII13MnII(μ2-O)6(μ3-O)6(μ4-PO4)2(μ5-PO4)2]5+ complex encapsulated by four [B-α-PW9O34]9- trivacant Keggin-type moieties [crystal data for 1: monoclinic, C2/c (No. 15), a = 36.341(7) Å, b = 18.325(4) Å, c = 36.668(7) Å, β = 119.24(3)°, V = 21308(7) Å3 Z = 4]. In a magnetic point of view, all of these paramagnetic manganese aggregates in 1 are well wrapped into the diamagnetic inorganic shells. Variable-temperature, solid-state, direct-current susceptibility measurements show that compound 1 exhibits strong antiferromagnetic interactions inside the [Mn14] core. [ABSTRACT FROM AUTHOR]