The aims of the study were to investigate the physicochemical properties, such as the phase equilibria (by the means of thermal analysis), density (by Archimedean method of hydrostatic weighing), viscosity (by torsion pendulum method) and surface tension (by maximum bubble pressure method), of the (LiF + NaF + KF(eut.) + Na7Zr6F31) fluoride system. The obtained data show significant deviation from the ideal/additive behaviour, particularly in the concentration range from (0 to 20) mol% of Na7Zr6F31 while the data sets correspond each other very well. In this range a sharp increase in temperature of primary crystallisation together with volume expansion, positive deviation from "additive" behaviour of viscosity and negative deviation from "additive" behaviour of surface tension were observed. These significant changes are discussed and indicate complicated interactions of the components in the melt. (C) 2014 Elsevier Ltd. All rights reserved.