Sum frequency generation (SFG) spectroscopy has been applied to study the electrochemical reduction of 4-(4-diethylamino)styryl)-N-methylpyridinium iodide (D289) at the electrolyte-Si(111) interface. By employing electrically conductive Si(111) as the substrate and choosing the proper excitation symmetry, the undesired background signal from the substrate can be eliminated completely without sacrificing the capability of creating an electrical double layer. Thus, a nonlinear optical window has been opened for the study of the chemical processes inside the electrical double layer. For the first time, many physical and chemical processes taking place at solid-liquid interfaces can be characterized with unprecedented molecular level sensitivity.