This work describes the theoretical analysis, experimental development, and application of an in situ spectroelectrochemical cell for measuring infrared reflection-absorption spectra (IRRAS) of monolayer-coated Au surfaces beneath a thin aqueous overlayer. The cell allows the interchange of sample and reference substrates, thereby facilitating IRRAS of long-chain alkanethiolates in the C-H stretching region without electrochemical or polarization-modulation methods;Using classical electromagnetic theory, the effects of angle of incidence, solution layer thickness, solution composition, and the window material on the observed in situ spectrum are examined. Experimentally determined values of optical functions, n and k, of frequency are presented for several materials used to test the in situ cell and the theoretical models. Calculations of both mean-square electric fields and IRRAS for a monolayer of octadecanethiolate on Au are used to identify optical artifacts and conditions to achieve high surface detectability. Experimental spectra for the same monolayer test these predictions and identify practical limitations of the technique;The same in situ cell is combined with electrochemical modulation to study monolayers self-assembled from 11-mercaptoundecyl ferrocenecarboxylate (FcCOOC[subscript]11SH) at gold and to correlate the monolayer structure and composition with the ferrocenyl redox chemistry. The features in the differential (voltage-dependent) spectra of the film are ascribed to changes in the bond strengths of the adsorbate resulting from oxidization of the ferrocenyl end group. No changes of the orientation of the polymethylene chains with applied voltage are detectable. Vibrational mode assignments for FcCOOC[subscript]11SH, based on studies of several analogs with different alkoxy groups, are presented--along with IR spectra and band assignments for the analogs;A novel electrochemically active monolayer of an alkanethiolate derivatized with a flavin, formed by self-assembly from ethanolic solutions of 11,11[superscript]'-dithiobisundecanoic acid, bis (2-(7,8-dimethyl-10-isoalloxazino)ethyl) ester, is described. The method of synthesizing the adsorbate precursor is included. Films formed from this compound display the rapid and pH-dependent electrochemical behavior typical of immobilized flavins; however, the films are quite stable and display coverages up to ~3 x 10[superscript]-10 mol/cm[superscript]2. IRRAS of the film, which is a candidate for future in situ spectroscopic studies, are presented and compared with transmission spectra of bulk analogs.