The effects of pH in the range 6.0-8.0 on the thermodynamics of the reversible thermal unfolding of Escherichia coli thioredoxin in the oxidized state have been determined over a range of concentrations using differential scanning calorimetry. The thermal denaturation indicated an inverse temperature dependence on concentration. The data were shown to fit a model based on dimerization of both the native and denatured states of the protein. The degree of dimerization of both states was found to be pH dependent. The previously described importance of protonation of the anomalously titrating aspartic acid 26 residue [Langsetmo, K., Fuchs, J., & Woodward, C. (1991) Biochemistry 30 ,7603-7609] was apparently verified by the agreement between the experimentally determined delta delta Gzerod and the calculated delta delta GzeroH in the pH range 7.0-8.0.