Abstract Explorations on a mixed metal phosphatoantimonate system led to the discovery of two new anhydrous phosphatoantimonates, namely, Cs₄MSb₆P₄O₂₈ (M=Mg, Zn), which represent the first examples of quinary A^I – M^II –Sb^V – P^V– O systems. Single-crystal structure analysis reveals that the two title compounds crystallize in the tetragonal space group I4₁/a (No. 88). Their structures feature complicated 3D frameworks with interesting tunnel structures comprised of corner sharing MO₄ tetrahedra, SbO₆ octahedra and PO₄ tetrahedra, with the Cs⁺ cations sitting in the tunnels to balance the valence. Optical reflectance spectrum measurements show that these two compounds are insulators with band gaps of about 4.5 eV.