Current heat transfer fluids for concentrated solar power applications are limited by their high temperature stability. Other fluids that are capable of operating at high temperatures have very high melting points. The present work is aimed at characterizing potential solar heat transfer fluid candidates that are likely to be thermally stable (up to 500 °C) with a lower melting point (∼100 °C). Binary and ternary mixtures of nitrates have the potential for being such heat transfer fluids. To characterize such eutectic media, both experimental measurements and analytical methods resulting in phase diagrams and other properties of the fluids are essential. Solidus and liquidus data have been determined using a differential scanning calorimeter over the range the compositions for each salt system and mathematical models have been derived using Gibbs Energy minimization. The Gibbs models presented in this paper sufficiently fit the experimental results as well as providing accurate predictions of the eutectic compositions and temperatures for each system. The methods developed here are expected to have broader implications in the identification of optimizing new heat transfer fluids for a wide range of applications, including solar thermal power systems.