The results of the study are presented, showing that the mechanism of superplastic deformation of industrial titanium and nickel alloys is dynamic recrystallization (fluctuation-structural transition), which leads to a steady segmental (vacancy-segmental) flow of the material when the sample is stretched. The results obtained confirm the fallacy of the concept of the relationship between grain boundary sliding and the motion of grain boundary dislocations during superplastic deformation and the doubtfulness of the physical validity of the velocity sensitivity coefficient as a criterion for superplasticity. Keywords: superplasticity, grain boundary sliding, dynamic recrystallization, fluctuation-structural transition, velocity sensitivity coefficient, polycrystalline superplastic viscosity. Onischenko-ak@uecrus.com