Excess heat capacity changes during the thermal unfolding of Kunitz-type soybean trypsin inhibitor were measured in an adiabatic differential heat capacity calorimeter at pH 7.0 at various heating rates. It was found that, unlike most globular proteins, the thermal unfolding was kinetically limited even at the heating rate of 0.5 K min(-1), The apparent enthalpy change of unfolding observed with various buffers was almost independent of the buffer ionization heat of each buffer component. The number of protons released from the protein during unfolding was 0.18 +/- 0.21, The van’t Hoff enthalpy change was estimated on the basis of an ideal two-state transition model from the DSC tracings which were observed at a heating rate slower than 0.25 K min(-1) and was found to agree with the calorimetric enthalpy within a deviation of 10%. A curve fitting analysis showed that the DSC signals fitted well to a two-state unfolding model. The net enthalpy change of unfolding was determined to be Delta H-0 = 429 +/- 6 kJ mol(-1) or 200 +/- 0.3 J g(-1) at 59 degrees C. The corresponding heat capacity change was Delta C-p(d) = 11.0 +/- 0.5 kJ K-1 mol(-1) or 0.50 +/- 0.04 JK(-1) g(-1).