Phase relations in the system CsAlF4-KAlF4 and Cs3AlF6-K3AlF6 were investigated by DTA and XRD methods. In the system CsAlF4-KAlF4, two intermediate compounds 3CsAlF(4). KAlF4 and CsAlF4. KAlF4 were confirmed. The first one was indexed as orthorhombic with a = 9.079 +/- 0.003, b = 7.958 +/- 0.002, c = 7.648 +/-0.004 Angstrom. The second one alpha-CsAlF4. KAlF4 is also orthorhombic, a = 10.982 +/- 0.005, b = 10.093 +/- 0.006, c = 6.160 +/- 0.002 Angstrom. Among the two compounds and KAlF4, two eutectic temperatures e(1) and e(2) are formed. The first eutectic temperature e(1) is located in 42 mol % KAlF4 at 525 degrees C, and e(2) in 70 mol % KAlF4 at 510 degrees C. The system Cs3AlF6-K3AlF6 is a successive solid solution system with the lowest melting point, the composition of which is 75 mol % Cs3AlF6 at 775 degrees C. In the solid solution, compound 2Cs(3)AlF(4). K3AlF6 was formed at 656 degrees C, and was indexed as a cubic cell with a = 8.891 Angstrom.