Enthalpy increment measurements on melts in the system KCl–K2TaF7 were carried out by drop calorimetry at temperatures between 298 and 1053, 1093 and 1133K and selected compositions. The heat capacities of melted mixtures and enthalpies of mixing were determined using the experimental data. Calorimetric experiments showed nonideality of the melts in several properties. The molar relative enthalpy and molar heat capacity of melts exhibited small negative divergence from additivity. The molar enthalpy of mixing of melts showed exothermic deviation from ideality which increased with increasing temperature. The thermal effect at mixing was assigned predominantly to addition and substitution reactions producing heteroligand complex anions TaF6Cl2−. The nonideality was also confirmed by evaluating of activities considering Margules type model for excess Gibbs energy of mixing.