The fusibility and liquidus temperatures of 10 compositions of the quaternary system K2ZrF6–K2NbF7–KF–KCl were studied in the ranges of component concentrations (mol %): K2ZrF6 – (1.73–4.03), K2NbF7 – (2.33–4.55), KF – (48.62–65.37), KCl – (30.57–42.83). The studies were carried out by the method of differential thermal analysis in the cooling mode. The temperature dependence on the molar ratios of the fluoride components of the melted phase is revealed. The most low-melting compositions are characterized by low values of the molar ratio of fluorozirconate to potassium fluoroniobate and, at the same time, increased values of the molar ratio of potassium fluoride to the sum of moles of zirconium and niobium salts. Their values were 0.52–0.56 and 10.5–13.7, respectively. The liquidus temperatures of these compositions are in the range of 854–873 K. High melting temperatures are characteristic of melts that are characterized by the highest values of the molar ratio of potassium fluorozirconate to potassium fluoroniobate at low values of the molar ratio of potassium fluoride to the sum of moles of zirconium and niobium salts. They were 5.4–7.9 and 0.8–1.1, respectively. The liquidus temperature of these compositions is in the range of 920–974 K.