Crystals of two polymorphs of CsCuPO₄ have been obtained by high-temperature reaction of CsNO₃, Cu₂(NO₃)2 and (NH₄)4P₂O₇. The structures of both polymorphs were solved using single crystal X-ray diff raction. α-CsCuPO₄ is orthorhombic, space group Cmcm, a = 7,0176(10), b = 6,8494(5), c = 22,691(2) A, V = 1064,1(4) A3, R1 = 0,116 for 782 unique reflections [I > 2s(I)]. The structure is based upon anionic layers [Cu(PO₄)]-, with Cs+ ions in between. β-CsCuPO₄ is orthorhombic as well, space group Pm21n, a = 5,3362(9), b = 8,8712(20), c = 8,967(18) A, V = 422,58(15) A3, R1 = 0,096 for 1191 unique reflections [I > 2s(I)]. The structure consists of the [Cu(PO₄)]-framework of the ABW topology formed by corner sharing of [PO₄] and [Cu2+O₄] tetrahedra. Cs+ ions reside in framework cavities. Comparative study of complexity parameters and molar volumes of the two phases demonstrates that the layered α-phase is a high-temperature modification, whereas the β-phase is a low-temperature modification. Refs 22. Tables 6.