Potassium fluoride tetrahydrate is of interest as a thermal energy storage material, due to its large specific and volumetric enthalpy of fusion and its low melting temperature. Here, we report the thermophysical properties of solid and liquid potassium fluoride tetrahydrate at temperatures from (243 to 348) K and compare this compound to water and octadecane, two other potential thermal energy storage materials with similar melting temperatures. Furthermore, we present a modified potassium fluoride-water phase diagram and accurately determine the enthalpies of fusion and melting temperatures for potassium fluoride tetrahydrate, Delta H-fus = (246 +/- 2) J.g(-1) and T-fus = 291.6 K, and the potassium fluoride tetrahydrate-potassium fluoride dehydrate eutectic, Delta H-fus = (203 +/- 2) J.g(-1) and T-fus = 282.2 K.