Density of fused mixtures of NaCl-KCl-ZrCl4 is studied by the method of maximum pressure in the gas bubble. The temperature dependence of density is given in the form of coefficients of linear equations. Variation of density is being governed by ZrF4 content in total concentration range. The mole volumes of all salt mixtures and their relative deviations from additivity for 1200 K are computed to reveal a character of interaction between components. The most polarizing and complexing capabilities of ZrF4 result in formation of complex anions of tetravalent Zr that exerts characteristic influence on volume properties of melts.