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The solubility of baddeleyite (ZrO₂) and the speciation of zirconium have been investigated in HF-bearing aqueous solutions at temperatures up to 400°C and pressures up to 700bar. The data obtained suggest that in HF-bearing solutions zirconium is transported mainly in the form of the hydroxyfluoride species ZrF(OH)₃° and ZrF₂(OH)₂°. Formation constants determined for these species (Zr⁴⁺+nF⁻+mOH⁻=ZrF_n(OH)_m°) range from 43.7 at 100°C to 46.41 at 400°C for ZrF(OH)₃°, and from 37.25 at 100°C to 43.88 at 400°C for ZrF₂(OH)₂°. Although the solubility of ZrO₂ is retrograde with respect to temperature, the measured concentrations of Zr are orders of magnitude higher than those predicted from theoretical extrapolations based on simple fluoride species (ZrF³⁺–ZrF₆²⁻). Model calculations performed for zircon show that zirconium can be transported by aqueous fluids in concentrations sufficient to account for …