The isobaric heat capacity in non-reacting and in reacting systems has been analysed. The isobaric heat capacity Cp in non-reacting systems depends on continuous distribution of enthalpy levels, according to the statistical thermodynamic model. The heat capacity in non-reacting systems is measured by supplying heat to the system and is related to the distribution of molecules either among enthalpy levels or among entropy configurations without any possibility of distinction. The isobaric heat capacity *Cp, app in reacting systems is related to changes in distribution of molecules among enthalpy levels separated by well defined large differences DH as assigned by the model. Therefore, the isobaric heat capacity is apparent because it depends also on the heat intrinsically produced by the reaction inside the system. The heat capacity DCp, app produced by changes of temperature in differential scanning calorimetry experiments is bound to changes of association fraction a(T) inside the system. It is shown how the denaturation heat determined by integration of the experimental curve can be reproduced by integration of the function of the association fractions in a two-state denaturation process. The treatment has been extended to macromolecules constituted by different domains giving origin to more complex profiles at the denaturation.