High temperature (HT) powder X-ray diffraction measurements and dilatometric investigations were carried out on microcrystalline NaZnPO4, KZnPO4 and TlZnPO4. The alpha- or room temperature (RT)-NaZnPO4 [1] with beryllonite-type (NaBePO4 [2]) structure undergoes a phase transition at 888 degreesC to a kalsilite (KAlSiO4 [3, 4])-like structure with the lattice constants a = 5.101(3) Angstrom, c = 8.365(8) Angstrom (at 920 degreesC), space group P 6(3), Z = 2. A distorted kalsilite-type structure (a = 8.748(7) Angstrom, c = 8,054(7) Angstrom, space group P 6(3), Z = 6) is formed, when pellets of NaZnPO4 were quenched from about 950 degreesC to room temperature. The alpha- or RT-KZnPO4 [5] transforms reconstructively at 683 degreesC from alpha -KAlGeO4-type [6] structure to the Icmm-type structure (a = 9.1480 (7) Angstrom, b = 53459(4) Angstrom, c = 8.7164(6) Angstrom at 740 degreesC, space group P c 2(1) n, Z = 4), in accordance with [7]. The RT (alpha) [8] and the HT-forms of TlZnPO4 show Icmm-topology structures. The lattice constants of beta -TlZnPO4 at 300 degreesC are a = 9.1272(7) Angstrom, b = 5.4478(5) Angstrom, c = 8.6807(7) Angstrom, space group P c 21 n, Z = 4. The values for the gamma -polymorph at 500 degreesC are a = 9.3203(6) Angstrom, b = 5 4207(4) Angstrom, c = 8.6988(6) Angstrom, space group P c m n, Z = 4, Topological and structural chemical aspects of the MZnPO4 compounds (M = Na, K, TI) and their polymorphs will be discussed.