Abstract: Differential scanning calorimetry was used to investigate the thermal denaturation of the proteinase K (EC 3.4.21.14) in the temperature range from 310 to 450 K. The influence of hydration and pH upon the thermal denaturation was studied in the range of total water content from 0.048 to 4.39 g H_2O/g dry enzyme and in the pH range from 3.29 to 8.49, respectively. The dependence of the denaturational temperature T_d, the specific enthalpy of denaturation ΔH_d and the maximum of excess apparent specific heat C_ex~(max) upon the degree of hydration h and the pH of the buffer is presented respectively. The relationship between the denaturational temperature T_d and the water content is of the Flory-Garrett's type. Besides this, the thermodynamic characteristics of the thermal denaturation of proteinase K are different from those reported of globular proteins, double-stranded coiled-coils and triple-stranded coiled-coils. The influence of primary hydration water upon the thermodynamic parameters of the thermal denaturation is the greatest. The external variable, the pH of the buffer used to dissolve the enzyme influences its thermostability to some extent.