The optical properties of a thin-layer electrochemical cell have been quantitatively analyzed with special consideration given to optimizing the experimental sensitivity of infrared reflection-absorption spectroscopy (IRRAS) to the typically weak infrared absorption due to an adsorbate on the working electrode surface. It is found that the sensitivity is dramatically affected by interference effects in the cell, and that such effects may be exploited to increase the fractional change in reflectivity ( ΔR R) due to the adsorbate. In addition, these effects strongly influence the lineshapes in vibrational spectra and can even reverse the sign of the reflectivity change. Some of the predicted optical properties have been experimentally observed using a thin-layer electrochemical cell and a tunable infrared laser to study CO adsorbed on an Pt electrode. © 1988.