PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) is a well-known membrane material showing good gas separation properties. The incorporation of nanoparticles can enhance or deteriorate the performance of composite membranes, sometimes depending only on the way of the composite preparation. We have modified the PPO polymer with C-60 fullerenes up to a content of 2 wt %. Previous investigations showed a strong dependence of permeability on whether the C-60 is simply dispersed in the polymer or chemically bonded to the polymer chains. Free volume effects were suggested as an explanation but not experimentally confirmed. Here, we present free volume studies by positron annihilation lifetime spectroscopy. An additional long positron lifetime shows the increased free volume of composite samples, while the high electron affinity Of C-60 helps to indicate the homogeneity of the samples. Combining the presented results with permeability measurements refines the understanding of this promising membrane material.