Physico-chemical interaction in the TlPb2Br5–КPb2Br5 and TlPb2Br5–RbPb2Br5 systems was investigated by XRD and differential thermal analysis methods. Lattice parameters of the samples were calculated using WinCSD software package. Crystallographic parameters were refined using Rietveld method.Phase diagrams of the quasi-binary systems TlPb2Br5–МPb2Br5 (M = K, Rb) were constructed using physico-chemical analysis methods (XRD, DTA). The investigated systems are of eutectic type with the eutectic point coordinates 73 mol.% KPb2Br5, 645 K and 71 mol.% RbPb2Br5, 633 K. The TlPb2Br5–КPb2Br5 system is characterized by the formation below the temperature of the phase transition of TlPb2Br5 of a continuous solid solutions series with the monoclinic structure (S.G. Р21/с). The TlPb2Br5–RbPb2Br5 system features two solid solution ranges, of the low-temperature modification of TlPb2Br5 with the monoclinic structure (S.G. Р21/с) extending to 57 mol.%, and of RbPb2Br5 with the tetragonal structure (S.G. I4/mcm) extending to 21 mol.%.The K0.45Tl0.55Pb2Br5 single crystal was grown by the solution-melt method at a Bridgman-Stockbarger set-up. The upper and lower zone temperatures were 710 K and 570 K, respectively. The temperature gradient at the solid-melt interface was 12 K/cm, and the lowering rate of the container was 8 mm/day.The quaternary phase K0.45Tl0.55Pb2Br5 crystallizes in the NH4Pb2Cl5 structure type (S.G. Р21/с, a=0.92745(8), b=0.83504(7), c= 1.3026(1) nm, β=89.90(2)°). It is formed by the isovalent substitution of thallium atoms by potassium in the TlPb2Br5 structure. All atoms in the structure occupy the 4e sites. However, the partial occupation of the site with Tl and K atoms is 0.55 and 0.45, respectively. The position of K atoms corresponds to the defect position of Tl atoms in the TlPb2Br5 structure. The closest surrounding of Pb1 and Pb2 atoms of eight Br atoms corresponds to the trigonal prism with two additional atoms. Tl atoms coordinate nine Br atoms forming a trigonal prism with three additional atoms against the side faces. The coordination surrounding of K atoms has 6 Br atoms and a neighboring Tl atom forming a trigonal antiprism. This arrangement of the atoms may give rise to new properties of the quaternary phase K0.45Tl0.55Pb2Br5 compared to the original ternary compounds.