Relative enthalpies of melts in the system KF-KBF4 at mole fractions of KBF4 equal to 0.35, 0.42, 0.60 and 0.80 have been determined from drop-calorimetry measurements at temperatures up to 1082 K. The temperature dependences of relative enthalpies of melts have been found to be linear. The heat capacities of melted mixtures indicate a positive deviation from additivity. The enthalpy of mixing shows a negative deviation from ideality in the entire composition range of the system. The exothermic effect at mixing reaches its minimum of about -3 kJ mol(-1) at the composition around x(KBF4) = 0.7. Negative enthalpy of mixing is explained by forming of individual BF4- ions, following the breaking of B-F-B bridges in KBF4 melt during the mixing process.