An apparatus is presented for the investigation, by transmission Fourier transform infrared (FT-IR) spectroscopy, of molecular interactions between supercritical CO2 (SC-CO2) and thin films of various materials. This instrument allows us to record transmission FT-IR spectra under SC-CO2 conditions for pressures up to 12 MPa in the range 10-80 degreesC. Reproducible spectra can be recorded in the mid infrared ranges 1000-2100 and 2600-3450 cm(-1) wavenumber. The new apparatus is here used to record FT-IR spectra of films of either albumin or lysozyme exposed to SC-CO2. At increasing CO2 pressure, FT-IR spectra indicate a progressive change in the relative amount of alpha-helix, beta-sheet, and random coil in the protein secondary structure, specific interactions between CO2 and protein amino acids, and also interactions between CO2 and the amine groups in some amino acid residues. The results presented show that, limited to the proteins considered here, a significant secondary structure reorganization happens for pressures above 8-9 MPa. However, our results also indicate that the protein secondary structure is almost completely recovered when room conditions are restored. These experimental findings provide details about the molecular mechanisms at the basis of microbial inactivation and of decreased biological activity, observed when treating enzymes and hormones with SC-CO2. (C) 2002 Elsevier Science B.V. All rights reserved.