In this study, miscibility and dynamic mechanical properties of hydrogenated nitrile butadiene rubber (HNBR) and 3,9-bis{1,1-dimethyl-2-[beta-(3-tert-butyl-4-hydroxy-5-methylphenyl) propionyloxy]ethyl}-2,4,8,10-tetraoxaspiro[5,5]-undecane (AO-80) composites were investigated. The composites with low-AO-80 content showed the incomplete miscibility, and a part of AO-80 dissolved into HNBR, while the rest was dispersed into HNBR in the form of deep-submicron-sized microspheres based on the micromorphological and thermal analyses. However, with the increasing AO-80 content, the system became completely miscible. When blending with AO-80, the resulting composite exhibited the remarkably improved dynamic mechanical property. The loss tangent (tan delta) peak of the composites gradually shifted to room temperature with the increasing AO-80 content. In this work, two classical equations (Gorden-Taylor and Fox) were used to predict the glass transition temperature of the HNBR/AO-80 composites. The calculated value based on Fox equation was more close to the experimental data. POLYM. ENG. SCI., 50:2375-2381, 2010. (C) 2010 Society of Plastics Engineers