In these first vibrational sum frequency spectroscopy studies of the CaF2/H2O interface conducted under varied aqueous phase composition, large changes in the water structure and surface composition have been found. As the pH of the aqueous phase is adjusted from acidic to basic, dramatic changes in the orientation and hydrogen bonding structure of the water molecules are observed, in addition to the clear presence of surface hydroxide species at high pH. Fundamental studies of the pH-dependent behavior of hydrous fluorite surfaces are essential for understanding the reaction mechanisms involved in calcium mineral flotation systems.