The coolant salt mixture: KF–NaF–ThF4 has been identified as a breeding blanket for the molten salt reactor. KF–NaF–ThF4 system is characterized with single quaternary fluoride, KNaThF6(s) which was synthesized by solution route. The heat capacity and Gibbs energy formation of KNaThF6(s) have been experimentally measured by differential scanning calorimeter and solid electrolyte Galvanic cell, respectively. The thermodynamic functions for KNaThF6(s) have been determined from measured data in this study along with required data of the literature.