Hindered phenols have been reported effective to remarkably enhance the damping property of acrylic polymers. In this work, the long-term stability of these novel damping materials were investigated on the basis of thermodynamic phase diagrams. The Flory-Huggins interaction parameter. between different small molecules and poly(butyl methacrylate) (PBMA) were determined through the melting point depression method proposed by Nishi and Wang. The calculated phase diagram curves agreed well with small-angle light scattering, wide-angle X-ray diffraction, and optical transparency measurements.. gradually decreased to a negative value as temperature increased and the phase diagrams exhbited an upper critical solution temperature-type spinodal curve. The mixtures with the small-molecule loading of less than 28 vol% were found miscibile at temperatures near the glass-transition temperature. Increasing the intermolecular hydrogen bonding strength between the phenol and carbonyl groups of PBMA could effectively reduce the. value. Meanwhile, increasing the steric hindrance of the phenolic hydroxyl groups and the sizes of small molecules may sufficiently weaken intramolecular hydrogen bonding interactions. These effects suppress the self-aggregation of small molecules and improve the long-term stability of the damping materials.