In situ reflection infrared spectroscopy with electrochemical modulation has been used to investigate the structure of the double layer for the system : Pt(hkl)/acetonitrile. The electrolytes used were tetraethylammonium perchlorate and sodium perchlorate. It has been found that acetonitrile is preferentially chemisorbed on the surface either through the non-bonding electrons on the nitrogen atom at potentials positive of the point of zero charge, or on its side in a rehybridized form at negative potentials. The transition between these orientations can be followed by the present technique. The experiments were also used to study the accumulation of solvated cations in the double layer. The spectroscopic data are discussed with respect to the orientation of the solvent and location of the electrolyte ions in the double layer.