Three new long-chain polyphosphates of yttrium AIY(PO3)4 (AI = Rb, Tl, Cs) were synthesized by heating stoichiometric mixtures of alkaline or thallous carbonates A2CO3, yttrium oxide Y2O3, and ammonium dihydrogen phosphate in a platinum crucible at temperatures ranging from 400 °C (Rb and Tl) to 450 °C (Cs). Their crystal structures were solved from single-crystal X-ray diffraction data. All colorless compounds are isotypic and belong to the type IV of the long-chain polyphosphates (monoclinic, P21/n, No. 14). The crystal structure is built of infinite helical chains of corner-sharing PO4 tetrahedra with a repetition unit of eight tetrahedra. These chains are further linked by corner sharing to isolated YO8 square antiprisms, thus forming a three-dimensional framework. The ions AI are located in hexagonally shaped channels running along [100] and are surrounded by eleven oxygen atoms in distorted environments. The luminescence properties of Eu3+ doped samples were investigated and used as structural probe in these polyphosphates. They are proving to be in good agreement with the crystallographic data.