Raman spectra have been measured for aqueous ZnSO₄ solutions under hydrothermal conditions at steam saturation to 244°C; solubility has been recorded as a function of temperature from 25 to 256°C. The high-temperature Raman spectra contained two polarized bands, which suggest that a second sulfato complex, possibly bidentate, is formed in solution, in addition to the 1:1 zinc(II) sulfato complex, which is the only ion pair identified at lower temperatures. Under hydrothermal conditions, it was possible to observe the hydrolysis of the zinc(II) aquo ion by measuring the relative intensity of bands due to SO ₄ ^2– and HSO ₄ ^– according to the equilibrium reaction Zn(OH₂)₆]²⁺ + SO ₄ ^2– [Zn(OH₂)₅OH]⁺ + HSO ₄ ^– The precipitate in equilibrium with the solution at 210°C could be characterized as ZnSO₄ · H₂O (gunningite) by x-ray diffraction (XRD) and Raman and infrared spectroscopy. At 244°C the equilibrium precipitate could be identified as ZnSO₄ (zincosite).