Binary data for the ruthenium(III) nitrosylnitrate obtained on a molality range from 0 to 18 mol kg−1 are reported. Due to the complex chemistry of this electrolyte, a combination of methods such as the simple solution concept and the Kusik-Meissner-Tester (KMT) method has been used. The necessity to use a ternary solution with the addition of nitric acid has also led to the determination of the deviation from the simple behavior of the RuNO(NO3)3/HNO3/H2O mixture. As no methodology existed prior this work to determine the binary data of an electrolyte presenting a deviation from the simple behavior when using the simple solution concept, this work brings improvements to this concept to adapt to such cases. Thus, the deviation has been estimated thanks to a mathematical approach and the Vdovenko and Ryazanov equation used for the computation of the density has been corrected to allow its use for mixtures with deviation from the simple behavior. Finally, the KMT method has been used to check if the results obtained are in accordance with what is usually observed in the literature for electrolytes.