The interfacial behaviour of water molecules at a polycrystalline gold electrode surface in a series of aqueous sodium halide solutions (NaX, X=F, Cl, Br and I) was investigated by in situ Fourier transform infrared reflection absorption spectroscopy. Sharp absorption bands due to OH stretching modes were observed in the 3600-3500cm (-1) range for each solution, containing Cl-,Br- or I-. The bands revealed almost no potential-dependent frequency shift but strongly depended upon the kind of electrolyte anion, which strongly suggests that these water molecules should be in the hydration spheres of the anions. The potential dependence of the band intensity was found to be associated with adsorption/desorption processes of anions onto/from the gold electrode surface, which was supported by double-layer capacitance measurements. The results were also compared with the behaviour in sodium perchlorate solution.