The subtractively normalized interfacial Fourier transform infrared technique has been employed to study the adsorption of benzonitrile (BN) at the Au(lll) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed BN molecules at positive potentials. Our studies show that BN molecules are totally desorbed from the Au(lll) surface at potentials more negative than -0.6 V (SCE) and they adsorb at the gold surface at more positive potentials. At potentials more negative than 0.05 V (SCE), the adsorption has an associative character. The BN molecules are initially oriented flat (pi-bonded) on the electrode surface and progressively reorient from the flat to a vertical (N-bonded) state when the electrode potential approaches the potential of zero charge. This change of the surface coordination is gradual and apparently involves a progressive change of the tilt angle. When the potential is greater than 0.05 V (SCE), the character of BN adsorption becomes dissociative and the adsorbed molecules partially hydrolyze to form benzamide (BA). The adsorbed layer becomes a mixture of BN and BA molecules. The ratio of BN to BA molecules decreases as the electrode potential increases. The optical properties of CaF2 prisms and flat windows have also been investigated. Our results show that flat windows should not be used to study physical features such as orientation and coordination of adsorbates. However, they are useful to extract quantitative information about the surface concentration of adsorbed species and to determine the composition of the interfacial region.