Abstract: The phase diagrams of LiF-KCl-KBr and KF-NaCl-NaBr systems have been de'er-mined by visual polythermal method with the X-ray diffraction analysis of solid phases.The thermodynamical behaviour of the molten salt solutions of these systems are estimated on the basis of the data of phase diagrams.The thermodynamical behaviour of these recipioral-salt-pair containing systems is explained by the results of the computerized simulation using Monte Carlo method.