The thermal unfolding of human carbonic anhydrase II (HCAII) has been studied by circular dichroism, UV-vis spectrophotometry, and differential scanning calorimetry (DSC). Coincidence of aggregation and tertiary structure disruption as well as fitting of DSC data showed that a two-state model can properly explain thermal unfolding of HCAII. According to this model, the average values of T* (the temperature at which k = 1/60 s(-1)), Delta H (enthalpy), and Delta E(a) (activation energy) are equal to 335.8 K, 698.6 kJ.mol(-1), and 529.0 kJ.mol(-1), respectively.