Polycarbonate (PC) and phenoxy resin (PKFE) have similar structures derived from bisphenol A. PKFE resin possesses pendant hydroxyl groups along its backbone; instead of the carbonate carbonyl groups found in polycarbonate, PKFE contains 2-hydroxypropyl groups instead. This difference in structure significantly influences thermomechanical properties when these materials are blended with phenyltrisilanol polyhedral oligomeric silsesquioxanes (POSS). While POSS exhibits some level of solubility in the each of the polymer matrixes tested, the glass transition temperatures of phenoxy composites increase with incorporation of POSS, while the glass transition temperatures of polycarbonate composites decrease significantly. Addition of POSS to PC also results in embrittlement of the composites; POSS thus serves as an antiplasticizer in the PC/POSS system. In the case of PKFE/POSS composites, significant interactions are evidenced by the change in the shape of the infrared spectra for the C-H phenyl stretch. While nuclear magnetic resonance spectroscopy shows definite reactions occurring between polycarbonate and POSS, no reactions were detected with the phenoxy resins. The POSS nanoscale filler can act as a plasticizer or as a reinforcing agent, depending upon the availability of filler-polymer interactions within the system.