Diffusion of gases in polymers below the glass transition temperature, T-g, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon-13 PFG NMR measurements of [C-13]O-2 diffusion in polymer films at 298 K are performed. Data obtained in polymers with T-g above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below T-g is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below T-g requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231-235, 2010