Phase transitions of protein solutions which are of interest for biotechnological and biomedical purposes should be analyzed from the point of view of their interrelation with phase transitions of the structure of protein molecules on the basis of the state thermodynamic functions. This is suggested by the coupling of intra- and intermolecular interactions in protein systems. Temperature trends of the standard thermodynamic functions of native (N) and denatured (D) protein in solution have been considered within the framework of the concept of excessive melting functions that determine the thermodynamic affinity of protein molecules for solvent. Thermodynamic solution functions of protein in native (N) and denatured (D) states have been built on the basis of microcalorimetry data. The nature of enthalpy, entropy and free energy temperature changes has been shown to suggest a theoretical possibility of both higher and lower critical solution temperatures. Previously there were only experimental indications for some proteins under certain conditions (solvent composition, pH etc.), but thermodynamical interpretation was lacking. Furthermore, there appears a supercritical zone between higher and lower critical solution temperatures on the phase diagram. The zone is of open (two-phase) or closed (one-phase) type depending on the protein conformation – D or N, respectively. This applies to the range next to physiological temperatures and makes the description of supramolecular organization of protein solution possible under such conditions. Significance of the supercritical zone of protein phase states is determined by its probable contribution into self-regulation of the system in response to the changes in environmental conditions (temperature, salinity, acidity etc.).