The thermal denaturation of ribonuclease A has been studied by use of Fourier transform nuclear magnetic resonance by monitoring the imidazole C-2 proton resonances of the histidine residues as a function of temperature at pH 1.3. As the temperature is raised, a slow chemical exchange process results in the disappearance of the peaks corresponding to the native conformation and the appearance of a single peak corresponding to histidine in the denatured state. The disappearance of the native peaks is not simultaneous, implying that at least two regions of the molecule denature at different temperatures. Also, fast chemical exchange processes result in small chemical shifts that appear to be related to local conformational changes. The observed phenomena have been shown to be reversible by the measurement of absorbance at 278 nm, enzyme activity, and nuclear magnetic resonance spectroscopy. The results of this equilibrium study support a multistate denaturation mechanism for ribonuclease A at pH 1.3.