The infrared horizontal ATR technique was adapted to be applied for in situ reaction monitoring even at high pressure and high temperature. Different types of reactors and flow cells were built which can be used for recording IR ATR spectra at pressures up to 200 bar and temperatures up to 300 degrees C. The use of the horizontal ATR technique is shown by the following application examples: addition reaction of n-butyl isocyanate with butyric alcohol; investigation of the equilibrium of isocyanate, HCl and carbamic acid chloride at elevated pressure and temperature; monitoring the polycondensation of bifunctional alcohols and carbonic acids; recording spectra of polymer melts at 280 degrees C.