Ion adsorption at aqueous interfaces is crucial in many environmental and biological processes. In spite of this importance, experimental investigation has been challenging due to the requirement on surface sensitivity and selectivity. I will introduce how interfacial water and ion adsorption can be probed by using nonlinear optical and X-ray spectroscopy. Two different model membranes consisting of choline and amine groups were chosen to make positively charged boundaries at the air/water interface. We monitored interfacial water structure upon adsorption of halide anions (Cl- and I-). Both choline and amine groups are expected to give positive potential on air/water interface, however the adsorption of ions and water structure were very different between the two surfaces.