Low- and high-temperature heat capacities were measured for ternary series of synthetic, high structural state feldspars in the NaAlSi3O8–KAlSi3O8–CaAl2Si2O8 system that had been characterised by X-ray diffraction and transmission light microscopy. The compositions of the phases lie in the NaAlSi3O8-rich part of the system where the stability is less limited by the ternary miscibility gap than in other compositional fields. The heat capacities of the end-members had been measured in previous studies using the same calorimeters as in this study (relaxation calorimeter and differential scanning calorimeter). Below 300 K, all samples showed strong positive excess heat capacities of mixing giving rise to excess entropies of mixing up to ΔSex = 5.8 ± 1.6 J mol−1 K−1 at T = 298.15 K. Above 300 K, further contributions to the excess entropies of mixing do not appear to be significant. The non-ideal entropic mixing behaviour was described by a ternary asymmetric Margules model, resulting in the ternary interaction parameter WAbOrAnS = 93.9 J mol−1K−1.