We present a systematic experimental investigation on the infrared (IR) vibrational absorption spectroscopy of then‐Si/acetonitrile interface utilizing the attenuated total internal‐reflection geometry in the near‐infrared (1.1–5 &mgr;m) spectral region. The IR absorption of the interface has been isolated selectively by electromodulation, and studied as a function of modulation potential. The electrochemical behavior of the interface has been checked by current/voltage and impedance measurements. The IR spectra are composed of a broad background and various sharp vibrational peaks. The background can be analyzed as the sum of two contributions: (i) absorption by surface states at shorter wavelengths (<2 &mgr;m), (ii) free‐carrier absorption at longer wavelengths. The free‐carrier contribution is itself composed of a Drude‐like component (proportional to &lgr;3/2) and an interband component. The vibrational peaks can be ascribed to the C≡N, C–H, Si–H, and (Si–)O–H chemical bonds. The shapes and magnitudes of the C≡N and C–H peaks can be quantitatively understood in terms of displaced ions and acetonitrile molecules near the surface upon the electrode potential modulation. The shape of the C≡N peak also gives an indication of a weak interaction of the acetonitrile molecules with the electrode surface. The Si–H and (Si–)O–H peaks can be interpreted in terms of Stark effect modulation of the infrared absorption of these species. The shape of the O–H peak indicates the presence of nonequivalent sites at the interface. Upon the electrode aging and oxidation the magnitude of the Si–H peak decreases and the (Si–)O–H peak increases and correspondingly the surface‐state density increases which provides a directinsituphysicochemical information regarding the slow oxidation of the electrode surface.