We present a thorough characterization of the thermal properties and thermal reliability of the eutectic mixture of decanoic acid with tetradecanoic acid, as a phase change material (PCM) of potential interest for passive temperature control in buildings. From the temperature-composition binary phase diagram we found that the eutectic composition is 0.82±0.02mole fraction (78±2 mass%) decanoic acid. We thoroughly characterized the thermal properties of the eutectic mixture. The eutectic composition has a high latent heat of fusion ΔfusH=153±15Jg−1 and a melting temperature Tonset=20.5±1.5°C. The heat capacity measured as a function of temperature for the solid and liquid phases just below and above the melting point is 1.9 and 2.1±0.2JK−1g−1, respectively. The average value of the thermal conductivity of the solid phase measured between −33 and 9°C is κs=0.20±0.02Wm−1K−1 and for the liquid phase, the thermal conductivity is κl=0.23±0.03Wm−1K−1 for 28 and 38°C. The mixture has a good long-term thermal stability as indicated by negligible changes in ΔfusH and Tonset after 3000 melt–freeze cycles. The parameters determined in this work allow more accurate modeling and optimization of the behavior of the eutectic mixture in preparation for implementation as a thermal energy storage PCM.