High sensitivity differential scanning calorimetry (d.s.c.) and uv-visible spectrophotometry have been used to study the thermal unfolding of alpha-chymotrypsin in presence of calcium chloride at pH = 2.8, 3.4, 5.0, 7.0, and 8.2. Quantitative thermodynamic parameters accompanying the thermal transitions have been evaluated. In the absence of calcium ions, the thermal denaturation of a-chymotrypsin is a reversible process giving a ratio of the van't Hoff to calorimetric enthalpy of 0.92 at pH = 2.8. At pH values higher than 5.0, the thermal denaturations in the absence of calcium chloride were observed to be completely irreversible. In the presence of calcium chloride, alpha-chymotrypsin undergoes irreversible thermal denaturation and its thermal transitions are found to be scan-rate dependent fitting to the model N-2 --> I, yielding an average activation energy of (419 +/- 16) kJ (.) mol(-1) using different approaches at pH = 2.8. It is also observed that at pH 2.8 and 3.4, calcium reduces the transition temperature of the protein. However, at pH 5.0, 7.0, and 8.2, it stabilizes initially, and at higher concentrations the salt acts as a destabilizer of the native structure of alpha-chymotrypsin. The surface tension values of aqueous calcium chloride solutions were measured and it is observed that the role of surface tension of the medium is not dominant in providing thermal stability of this protein.