The following metantimonates and metarsenates have been found to be isomorphous: CaSb2O6, SrSb2O6, SrSb2O6, CdSb2O6, HgSb2O6, CaAs2)6, SrAs2O6, CdAs2O6, HgAs2O6, PbAs2O6, and CoAs2)6, all, except the last one, having comparatively large cations. They crystallize in D1/3--C 312 and their trigonal cells contain one formula unit. The structure is built up of (Sb2O6)2- sheets formed of coupled SbO6 octahedra, between which the bivalent cations are inserted. Hence the edge a of the unit cell changes very little within each of the antimonate and arsenate series whereas the height c of the cell varies greatly with the size of the bivalent cations.