The mechanical relaxation behavior and microstructure of a series of novel norbornyl-POSS organic-inorganic copolymers have been investigated. We have examined the influence on physical properties of both the weight, fraction of POSS-norbornyl monomer and the corner group composition. POSS. refers to the polyhedral oligomeric silsesquioxane inorganic/organic macromer, which is composed of an inorganic Si8O12 spherical core surrounded by seven inert organic corner groups and one reactive norbornyl moiety, It was observed that POSS copolymerization enhances the cr-relaxation temperature, T-alpha, in proportion to the weight fraction of the POSS-norbornyl comonomer. Interestingly, however, the magnitude of this this dependence is larger for the POSS-norbornyl comonomer possessing cyclohexyl corner groups (CyPOSS) than for the copolymer with cyclopentyl corner groups (CpPOSS). Although POSS copolymerization yields only slight enhancement of the room temperature storage modulus, at temperatures lower than a strong mechanical relaxation, identified as a beta-relaxation, and near T = -78 degrees C, there isa significant POSS reinforcement of the storage modulus, The position of the beta relaxation observed in the CyPOSS series of copolymers is Independent of POSS weight fraction, and the frequency dependence of this peak position yields the activation energy, Delta H-beta = 14.7 (liberation of motion of the cyclohexyl corner groups). A similar effect is observed, to some extent, for the CpPOSS copolymers, although only part of Be relaxation is observed. X-rag scattering shows that, the CyPOSS copolymerization preserves the amorphous character of the polynorbornene homopolymer but CpPOSS copolymerization leads to significant ordering of the POSS macromers.