According to basic phenomenological models describing the solution-diffusion based mechanism of penetrant diffusion in dense polymers, a connection between the diffusive transport of gas molecules in a polymeric matrix and the molecular mobility of that matrix on a certain length scale is, in principle, established for a long time. However, experimental data directly showing this correlation are rare. The investigation of a series of nanocomposites based on a polyhedral oligomeric silsesquioxane (POSS) and a polycarbonate matrix allows a systematic change of the molecular mobility on a local length scale (-relaxation) and of the corresponding activation energy E-A, both determined by broadband dielectric spectroscopy. Independently, activation energies of penetrant diffusion (E-D) of these nanocomposites were determined for N-2, O-2, CO2, and CH4 and a clear linear correlation between the two activation energies was established for the first time. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1593-1597