In-situ infrared-spectroscopic characterization of polymer brushes plays an important role for a deeper understanding of the brushes' structural and chemical properties during stimuli-induced switching experiments. In this work, we used infrared-spectroscopic ellipsometry (IR-SE) to investigate the temperature-dependent swelling behavior of poly(N-isopropylacrylamide) [PNIPAAm] brushes below and above their lower critical solution temperature. IR-SE spectra were evaluated using optical-layer calculations in order to determine swollen-brush thickness and water content inside the brush. For a d(dry)=12.6 nm thin PNIPAAm brush, we found swollen- and deswollen-brush thicknesses of respectively (43 +/- 8) nm and (20 +/- 9) cm, which is in good agreement with ellipsometry results in the visible (VIS) spectral range. We determined water contents of (86 +/- 13) % and (68 +/- 15) %, and by correlating IR-SE with VIS-ellipsometric measurements, we were even able to obtain values of up to 2% accuracy. (C) 2012 Elsevier B.V. All rights reserved.