The application of near infrared spectroscopy to the quantitative analysis of compressed gases and liquids including fluid phases at pressures up to 3.5 kbar and temperatures up to 350 °C is illustrated. The technique is very valuable for the on-line study of chemical transformations, chemical equilibria and phase equilibria in this extended fluid range. Examples will be presented of kinetic studies of high pressure polymerization where near infrared absorbance is partly measured with a time resolution of microseconds. It is also shown that by simultaneously recording the IR and NIR spectral ranges on an FT-instrument, the dynamic range for on-line concentration measurement may be considerably enhanced, and the course of reactions may be quantitatively followed from the initial stage up to complete conversion.