A comparative study of the thermal stabilities of five β-galactosidases of different origins in buffer solutions at pH of their highest activity was performed. The thermal inactivation of these enzymes was found to occur via different mechanisms. The thermal inactivation of four β-galactosidases followed the mechanism with intermediate stages not accompanied by catalytic activity loss. The dissociative mechanism of inactivation, including the reversible dissociation of the oligomeric enzyme and the irreversible dissociation of the monomeric enzyme, was observed for bacterial (Escherichia coli) and yeast (Kluyveromices fragilis) β-galactosidases. The kinetic parameters of dissociative thermal inactivation of these enzymes and the stability parameters of β-galactosidases studied were determined. The latter included the critical temperature of changes in the kinetic regime of inactivation, the smallest number of intermediate stages without catalytic activity loss, the temperature of the disappearance of the induction period of thermal inactivation, and induction period duration at the given temperature (40°C).