The present study demonstrates a real-time ATR-FTIR spectroscopic monitoring of the PSU polycondensation reaction. The analytical interpretation of the real-time IR spectra with respect to different reaction steps was carried out on the basis of characteristic vibration bands of monomers and PSU. Moreover, the tracing of the polycondensation reaction in reference to the generated concentration of PSU and averaged molar masses were also under consideration. The molar masses of the synthesized low- and high-molecular PSUs determined by different analytical methods were compared. A selective calibration of PSU concentrations was calculated by a chemometric method and confirmed by two validation procedures. This calibration was used for the monitoring of PSU polycondensation reaction to get information on the kinetics of the described reaction. (c) 2006 Elsevier Ltd. All rights reserved.