Phase relations of the AlF3-CsF system have been investigated by the methods of DTA and XRD with quenching technique. Four compounds were identified : Cs3AlF6, CsAlF4, CsF . 2AlF(3) and CsF.3AlF(3). Cs3AlF6 melts congruently at 790 degrees C. The first eutectic, E-1, between Cs3AlF6 and CsF is located in 10.0 mol% AlF3 at 654 degrees C. CsF . 2AlF(3) and CsF . 3AlF(3) melt incongruently at 508 degrees and 653 degrees C, respectively. The second eutectic, E-2, was observed in 42.0 mol% AlF3 at 471 degrees C. The compound CsAlF4 formed in the solid eutectic when cooled below 443 degrees C. CsAlF4 has alpha and beta forms, transformation of which takes place reversibly at 422 degrees C. All phase structures in the system were confirmed by X-ray powder diffraction analysis.