The sequence and character of polymorphic transformations of the MB₅O₈ modifications (M = K and Rb) are investigated by glass crystallization, high-temperature X-ray diffraction, and differential thermal analysis (DTA). The Debye diffraction patterns are obtained for all the modifications, including the γ-KB₅O₈ and α-RbB₅O₈ compounds (not studied previously by X-ray diffraction). The symmetry and unit-cell parameters for the latter modification are determined. It is elucidated how the dispersity affects the phase composition and structural state of the substance. It is revealed that singular points in the temperature dependences of some unit-cell parameters and lattice volume of the metastable crystalline phases are close to the glass transition temperature of the melts. This fact suggests that thermal transformation processes in crystals and glasses of similar compositions have a common nature.