The solubility and speciation of the assemblage MoO
2–MoO
3 in water vapour were investigated in experiments conducted at 350 °C,
Ptotal from 59 to 160 bar and
fHCl from 0 to 3.4 bar (0–2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑
fMo from 4.4 × 10
−4 to 6.5 × 10
−2 bar). The concentration of Mo in the vapour at
fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at
fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at
fHCl below 0.1 bar, but increases with increasing
fHCl above this pressure. The dominant Mo species at
fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reaction