This study reports the measurements of the solubility of crystalline molybdenum trioxide, MoO3, in aqueous solutions of the mixture HCl-HClO4-NaCl (up to 1 m of chloride ion) at 573.2 K and pressures close to the saturation water vapor pressure by the autoclave method with the chemical analysis of quenched solutions. In total, there are 75 data points. The data treatment shows that the most probable chloride species of Mo(VI) in the studied solutions is MoO2(OH)(2)Cl- (the coordinated water is omitted), which agrees with the literature extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra studies of speciation of Mo(VI) in chloride solutions at elevated temperatures. For the solubility reaction MoO3(cr) + H2O(1) + Cl- = MoO2(OH)(2)Cl-, log(10) K degrees= (0.87 +/- 0.2).