The thermal unfolding of acid proteinase A, isolated from Aspergillus niger, was studied by differential adiabatic scanning calorimetry at various pi-Is ranging from 2 to 9. The temperature of the maximal excess heat capacity was strongly dependent on pH and highest at a pH around 3, whereas the enthalpy change of the unfolding was maximal at a pH around 5. The excess heat capacity curve at a pH below 6 showed a single asymmetric peak. In contrast, the unfolding at a pH above 6 exhibited an excess heat capacity curve which is characterized by two sharp and broad peaks. The broad peak observed at about 40 degrees C at a pH above 6 was found to be unchanged irrespective of the pH of the solution. Curve resolution revealed that the unfolding at a pH range between 5 and 6 is characterized by a simple two-state transition, with dissociation to the light and heavy peptide chains.