The stability of a protein or of its folding intermediates is frequently characterized by its resistance to chemical and/or thermal denaturation. The folding/unfolding process is generally followed by spectroscopic methods such as absorbance, fluorescence, circular dichroism spectroscopy, etc. Here, we demonstrate a new method, by using HPLC, for determining the thermal unfolding transitions of disulfide-containing proteins and their structured. folding intermediates. The thermal transitions of a model protein, ribonuclease A (RNase A), and a recently found unfolding intermediate of onconase (ONC), des [30-75], have been estimated by this method. Finally, the advantages of this method over traditional techniques are discussed by providing specific examples. (C) 2003 Elsevier Inc. All rights reserved.