This article reports an investigation into the miscibility and morphology of octaisobutyl-substituted polyhedral oligomeric silsesquioxane (ibPOSS) nanocages over a wide concentration range (25-75 wt%) in polymer matrices, produced by solution blending. Three different, strategically chosen polymers were used: polystyrene (PS), poly(isobutyl methacrylate) (PIBMA), and poly(methyl methacrylate) (PMMA), to allow the effect of matrix structure and polarity on miscibility to be investigated. Differential scanning calorimetry, dynamic mechanical analysis, X-ray diffraction, and scanning electron microscopy were used to characterize the composite systems. The T-g of PMMA and PIBMA were found to increase with introduction of ibPOSS, whereas PS did not show any significant change of T-g with increasing POSS content, indicating differing degrees of interaction. The phase separation has been found and a cubic POSS crystal structure has been observed in all blends with different size. The glass transition increased with concentration, even though all compositions were predominantly phase-separated, and we suggest that this is due to the influence of the external face of the crystalline units on the polymer chains, and possibly some polymer contained within the crystalline POSS phase. POLYM. ENG. SCI., 50:991-999, 2010. (C) 2009 Society of Plastics Engineers