Electrified interface between the Au(100) surface and an acetonitrile electrolyte has been studied with in-situ reflection/absorption infrared spectroscopy and capacitance measurements. Electrosorptlon of dimethylsulfoxide from the same electrolyte has also been investigated. In addition, the interaction of acetonitrile and dimethylsulfoxide with the gold surface in vacuo have been assessed with various surface science methods. Thereby, the work function change, binding energy, orientation and electronic state of the adsorbates, as a function of its coverage, have been determined. The data are used to scrutinize current models of the double layer at metal/nonaqueous electrolyte interfaces.