Polymers are known to exhibit strong rate-dependent mechanical behavior. In different temperature and/or frequency regimes, the rate sensitivities of polymers change as various primary (alpha) and secondary (beta) molecular mobility mechanisms are accessed. The incorporation of narroparticles into the polymer matrix can potentially alter the local molecular level structure and thus offers an opportunity to tailor the rate-dependent mechanical deformation and failure behavior of the polymer. In this study, methacryl-POSS is incorporated into poly(vinyl chloride), producing a range in weight fraction of well-dispersed POSS. Dioctyl phthalate (DOP) plasticized PVC is also prepared with a range in DOP content using the same method. Both methacryl-POSS and DOP plasticize the PVC. Dynamic mechanical analysis (DMA) revealed that the incorporation of POSS in PVC introduced reductions in both the primary (alpha) and secondary (beta) transition temperatures. DOP reduced the a-transition temperature in the blends, whereas a pronounced suppression of the beta-relaxation peak was observed at higher DOP content. The rate-dependent yield and postyield behavior are characterized in compression testing over a wide range of strain rates (10(-4)-3000/s). A clear rate-dependent transition associated with the strainrate-dependent relaxation of beta-motions was observed in the compression yield data of PVC/POSS blends. Such a transition was much milder in the case of PVC/DOP due to the suppression of the beta-transition in these blends.