We have studied the hydrophobic water/octadecyltrichlorosilane(OTS) interface using the phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS),and obtained detailed structural information of the interface at the molecular level.Excess ions emerging at the interface were detected by changes of the surface vibrational spectrum induced by the surface field created by the excess ions.Both hydronium (H3O+) and OH- ions were found to adsorb at the interface,and so were other negative ions such as Cl-.