Thermal denaturation of ribonuclease A (RNase A) in D2O solution is studied by Fourier transform infrared (FT-IR) spectroscopy. Sample-sample two-dimensional correlation (SS 2D) spectroscopy and principal component analysis (PCA) are applied to these spectral data to reveal the thermal kinetics of RNase A. The second scores plot of PCA constructed from temperature-dependent original IR spectra illustrates a pretransition at 46 degrees C as well as a clear main transition at 66 degrees C. The latter is revealed by the SS 2D correlation spectra and the first score of PCA because of their illustration of the main denaturation event of RNase A, while the former cannot. Therefore, the present study demonstrates the great potential of PCA in revealing subtle phase transition of proteins in aqueous solutions. (C) 2006 Elsevier B.V. All rights reserved.