The speed of sound in binary molten KCl–KBr, KCl–KI, KBr–KI, KF–KCl, KF–KBr, and KF–KI mixtures has been measured by the pulse-temporal method in wide temperature and composition ranges. The adiabatic compressibility and molecular speed of sound have been calculated based on the lite-rature data on density. The results are analyzed from the point of view of structural transformation in ionic melts, which accompanies the mixing of salts with different ion sizes.