This study reports the measurements of solubility of crystalline molybdenum trioxide, MoO3, in NaClO4 solutions at 573.2 K and pressures, close to the saturation water vapor. In total, there are 16 data points at NaClO4 molalities between 0.005 and 2.21. The solubility is explained by two dissolution reactions: MoO3(cr) + H2O(1) = HMoO4- + H+, and MoO3(cr) + H2O(1) = H2MoO4(aq). The solubility moderately increases with the sodium perchlorate concentrations. In our view, this effect can be fully explained by the variations of the activity coefficients of H+ and HMoO4- in sodium perchlorate solutions, without the need to invoke a complex formation between Na+ and HMoO4-, as was proposed in the literature. A better knowledge of speciation, association and thermodynamic properties of solutions of typical electrolytes (such as NaCl, NaClO4, etc.) at elevated temperatures is a prerequisite for a more definite and precise interpretation of solubility data for solid compounds in high-temperature solutions.