We have previously reported that high hydrostatic pressure protected two beta-galactosidases against thermal inactivation. Here, the Fourier transform infrared spectroscopy (FT-IR) technique was used to examine the pressure- and temperature-induced changes occurring in the structure of these two proteins. A thermostated diamond anvil cell allowed to follow in situ conformational changes against the variation of both temperature and pressure. FT-IR measurements have been carried out on proteins in D2O-based solutions. Using D2O, the amide I' band (N-deuterated) can be measured without solvent interference. Analysis of the reduction in amide II intensities, which is due to peptide H-D exchange upon heating of the Aspergillus oryzae beta-galactosidases in D2O, showed that the temperatures for which H-D exchange acceleration occurred were 55, 65 and 60degreesC at atmospheric pressure (0:1 MPa), 285 and 430 MPa, respectively. With the Escherichia coli beta-galactosidase, these temperatures were 49, 52 and 51degreesC at 0.1, 230 and 410MPa, respectively. Interestingly, for the two enzymes considered, these values correlate quite well with the temperatures for which an irreversible activity loss of 50% is observed following a 30 min incubation. The pressure- and temperature-induced spectral changes in the amide I/I' vibrational band of these two proteins are also discussed.