Potential-dependent reorientation of a water molecule, adsorption of sulfate, and interactions between water and sulfate on a highly ordered Au(111) electrode surface in sulfuric acid solutions have been investigated in situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. The spectrum of the water layer at the interface changes in both intensity and frequency as the applied potential changes due to the reorientation of water molecules. The orientations deduced from infrared spectra are in good agreement with the predictions made by molecular dynamics simulations at potentials below and around the potential of zero charge (pzc) of the electrode where sulfate adsorption is negligible. At potentials above the pzc, sulfate anion is adsorbed at S-fold hollow sites on the (111) surface via three oxygen atoms. When the potential is increased and the fractional coverage of sulfate reaches to about one-half of full coverage, adsorbed sulfate anions start to form short-ranged domains and greatly change the water layer structure. Water molecules stabilize the sulfate domains by bridging neighboring sulfate anions via hydrogen bonding. The weak tunneling spots observed in the reported scanning tunneling microscopy images of the well-ordered (root 3 x root 7) sulfate adlayers on (111) metal surfaces are attributed to water molecules that bridge adjacent adsorbed sulfate anions.