High-sensitivity differential scanning calorimetry has been applied to characterize the irreversible thermal denaturation of a cellulase, assuming that thermal denaturation takes place according to the kinetic scheme N-k-->D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N the native state, and D the denatured one. On the basis of this model, the values of the rate constant as a function of temperature and the activation energy were calculated. The analytical data obtained with the fluorescence method as well by measurement of the enzymatic activity temperature dependence support this two-state kinetic model.