Stabilization of polymers involves chemical and physical processes, and the effectiveness of antioxidants is strongly influenced by their compatibility with the host system. In the case of rubber-modified high-impact polymer blends, the dispersed elastomer phase is generally the most sensitive to thermooxidative degradation. The compatibility of five hindered phenolic antioxidants with polybutadiene (PBD) was studied as a function of temperature. Relationships were found between the diffusion rate and the specific volume of additive, as well as the total free-volume of the polymer/additive system. The solubility of antioxidants was evaluated on the basis of thermodynamic theories. Deviations from thermodynamic expectations are explained in terms of different physical states and self-association of the substances investigated.