A simple system for high‐pressure FTIR measurements was designed and tested. This system can be applied to physicochemical measurements andinsituanalysis of chemical reactions in supercritical fluids. The principle of the high‐pressure IR optical cell proposed in this study was based on that of the cylindrical internal reflectance. This system was tested by measuring the IR spectra of supercritical CO2and of naphthalene dissolved in supercritical CO2 .