Phase equilibrium theory and the laws of thermodynamics provide valuable tools for predicting the composition and the thermal properties of eutectic mixtures. In this work, we briefly review the predictions based on ideal mixtures. We evaluate the accuracy of the models by comparing the predicted values of the composition, the melting temperature (T-mpt) and the enthalpy change (Delta H-fus) of binary mixtures of fatty acid phase change materials (PCMs), specifically as measured for decanoic tetradecanoic acids, and dodecanoic tetradecanoic acids and with values reported in the literature for other fatty acid eutectics. Although the phase diagrams depart from the ideal system (e.g., peritectic), there is, overall, reasonable agreement with the experimental data, i.e., the thermodynamic models can be a good starting point for the prediction of eutectic behaviour in binary or higher order mixtures of organic PCMs. A computational tool based on these thermodynamic models was developed in Microsoft Excel Kahwaji and White, [1] and used to calculate the composition, T-mpt and Delta H-fus of a total of 105 binary mixtures formed from 15 different fatty acid PCMs. The calculated properties show that eutectics both extend the lower range of T-mpt and provide a wide range of new T-mpt values that are not otherwise available from individual fatty acids, hence considerably expanding the scope of applications of fatty acids as PCMs for thermal energy storage.