Polymers with intrinsic microporosity (PIMs) are of great interest in the field of gas separation membranes. Already the first synthesized PIM-1 shows extraordinary permeability and selectivity. Unfortunately, rma-i is susceptible to physical aging and thus gradually loses its outstanding properties. In this study a polyhedral oligomeric silsesquioxane with phenethyl substituents (PhenethylPOSS) was used as a nanofiller (0-40 wt%) in the PIM-1 matrix to potentially improve the gas transport properties and prevent physical aging. The molecular mobility of the solution-cast nanocomposite films was analyzed by Broadband Dielectric Spectroscopy (BDS). Furthermore, gas permeability was determined with the time lag method (0-20 bar upstream pressure) at 35 degrees C for N-2, O-2, CH4 and CO2.