The mechanism for hydrothermal growth of ZnO was studied in ammonium hydroxide solution at pH near 11 (0.3 M NH₄OH). The products formed at 20–90 °C and ambient pressure were characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photon Spectroscopy (XPS), X-ray Diffraction (XRD) and Secondary Ion Mass Spectroscopy (SIMS). Under these conditions, the growth of ZnO occurs via the initial precipitation of ε-Zn(OH)₂ (Wülfingite), which subsequently dehydrates, to form Würtzite ZnO. Isotope tracking experiments show that most of the oxygen atoms do not mix with water during the conversion from Wülfingite to ZnO, and thus that the reaction proceeds primarily in the solid phase rather than through dissolution then reprecipitation. XPS results show that the surface of hydrothermal ZnO consists primarily of zinc hydroxide at lower temperatures, and there is a significant fraction of zinc hydroxide persisting at higher temperatures. Together these results show that much of the solid-solution interface is mainly comprised of zinc hydroxide not ZnO. These findings may have implications for understanding how small organic molecules can be used to control the morphology of zinc oxide crystals grown under hydrothermal conditions.