This work focuses on the validity of the simple solution concept of the uranium(IV) nitrate and uranium(VI) nitrate aqueous mixtures through different means such as water activity or density measurements using a simple solution approach. Since these two elements are mixed together in the plutonium uranium redox extraction (PUREX) process during the partitioning step, which separates uranium from plutonium, the knowledge of behavior of this mixture will help in the modeling of this process. This will allow a more precise code to be developed and thus better modeling of the different components involved in the PUREX process. Lately, thermodynamic codes are developed taking into account deviation from ideality. These deviations from ideality are estimated thanks to a solution theory. The one that will be used in this paper is the simple solution approach, a simplification of the McKay and Perring equation. The simple behavior of the uranium(IV) and uranium(VI) mixture will be checked using different methods. The precision of each method will then be discussed. This will be made through the acquisition of water activity, density, and molality of different solutions of uranium(IV) nitrate and uranium(VI) nitrate.