A novel manganese(IV)-containing symmetrical heteropolyvanadate was prepared by the transformation of K-7 [MnV13O38]center dot 18H(2)O (K7MnV13) to K4Li2[MnV14O40]. 21H(2)O (1) at pH 4. The heteropolyanion [MnV14O40](6-)(MnV14) is composed of a MnO6 octahedron surrounded by 14 edge-sharing VO6 octahedra. The simplified representation of MnV14 has a new topology termed jbal with a total point symbol of {3(10)}2{3(14).4(7)}4{3(18),4(10)}8 {3(44).4(46).5}. In the crystal lattice of 1, MnV14 packs with potassium ions, forming a 2D layered K-2[MnV14O40](4-) network (ABABAB...). Four K+ ions cap the four square O-4 faces of MnV14, apparently stabilizing the heteropolyanion. Compound 1 catalyzes the t-BuOOH-based oxidation of 2-chloroethyl ethyl sulfide (a mustard gas simulant). The magnetic and catalytic properties of 1 are discussed.