This study reports the measurements of the solubility of crystalline molybdenum trioxide, MoO3, in acid (HClO4, HCl) aqueous solutions at 563, 573, 593, and 623 K, mostly at pressures close to the saturated water vapor pressure. The solubility data can be explained by the two dissolution reactions: a). MoO3(cr) + OH- = HMoO4-, with log(10)K(o) values equal to (7.08 +/- 0.20), (7.09 +/- 0.20), (7.43 +/- 0.20) at 563, 573, and 593 K, and b). MoO3(cr) + H2O(l) = H2MoO4(aq), with log(10)K(o) equal to -(2.40 +/- 0.20), -(2.22 +/- 0.20), -(2.41 +/- 0.20), -(1.95 +/- 0.20) at 563, 573, 593, and 623 K, respectively. Combining these results with the literature data on the thermodynamics of H2MoO4 in the ideal gas state, we evaluated Henry's constants of this form at investigated temperatures, which turned out to be close to those of Si(OH)(4), just as one would expect on the basis of chemical similarity. Reasonable estimates of the fugacity coefficients of H2MoO4 in steam allowed the calculation of the vapor-liquid distribution constant, View the K-D=lim(x -> 0)y/x, for this species, where y and x stand for the mole fractions of a solute in coexisting vapor and liquid phases, respectively. The value of the Krichevskii parameter, which governs the variation of K-D in the investigated range of temperatures, is equal to -(228 +/- 33) MPa for H2MoO4, being close to that of Si(OH)(4), -(190 +/- 10) MPa. (C) 2017 Elsevier B.V. All rights reserved.