When designing a concentrating solar power (CSP) system, selection of a proper heat transfer fluid (HTF) material is a key, especially when employed in parabolic trough CSP plants. In particular, the use of low melting mixtures as an alternative to the widely commonly used “solar salt” can increase the CSP manageably and, as a result, several innovative nitrite/nitrate mixtures have been proposed. However, very few thermodynamics data are available for these compounds, especially regarding ternary compositions. One of the most interesting low freezing mixture is prepared with sodium and potassium nitrate together with sodium nitrite. The aim of this work is to investigate the thermodynamics properties of this ternary system, starting from its binary subunits, studying the phase diagram of this compound both experimentally and by a regular solution model. At this purpose, the literature phase diagrams of the binary subsystem were simulated in order to obtain the fitting parameters necessary for the employed semi-predictive tool. Then, the ternary system was modeled and the results showed very good agreement with the experimental points. It is quite interesting to note that both the theoretical and experimental results showed a low melting zone presenting greater sodium nitrate molar fractions with respect to sodium nitrite than previously reported in literature. This would lead to a decrease in the HTF price and an improvement regarding the fluid toxicity.