The adsorption of chloride and bromide ions on a gold electrode in the double layer region in 0.5 M perchloric acid solution was observed 'in-situ' for the first time using the technique of synchrotron far infrared reflectance spectroscopy (SFIRS). The Au-Cl surface vibrational frequency was measured to be about 263 cm(-1) in chloride concentrations from 0.005 to 0.05 M. The Au-Br stretching mode was observed at 182 cm(-1). The SFIRS technique is a promising tool for the investigation of interfacial electrochemical structures and processes with monolayer sensitivity.