Phase relations in the systems KAlF4-Rb3AlF6 and KAlF4-Cs3AlF6 were investigated by DTA and XRD methods for the purpose of finding a low melting, non-corrosive aluminum flux. In each system, congruent compounds Rb3AlF6. 2KAlF(4) and 2Cs(3)AlF(6). 3KAlF(4) were observed, respectively. Eutectics located at 46 mol% Rb3AlF6 and 511 degrees C, 25 mol% Rb3AlF6 and 512 degrees C for the former system, and 48 mol% Cs3AlF6 and 550 degrees C, 25 mol% Cs3AlF6 and 480 degrees C in the latter were confirmed. The phase alpha-2Cs(3)AlF(6). 3KAlF(4) was XRD indexed as having an orthorhombic cell a=10.401+/-0.003, b=10.078+/-0.006, c=8.792+/-0.006 Angstrom.