Using solid-state NMR, site-specific chain dynamics from the hertz to mid-kilohertz regime were Using solid-state NMR, site-specific chain examined in compression-molded films of poly(ethylene terephthalate) (PET), PET filled with 2 wt % acetanilide or phenacetin, and PET annealed below and above the glass transition temperature (T(g)). More specifically, (13)C relaxation time in the rotating frame, (1)H-(13)C cross-polarization times, and (13)C chemical-shift anisotropy exchange were measured at natural abundance. (13)C CP/MAS spectra revealed that PET annealed above T(g) was 28% crystalline, and all other PET samples were completely amorphous. Relaxation times in these polymers were analyzed using stretched exponentials which yielded characteristic time constants. The filled PET and PET annealed above T(g) exhibited suppressed chain dynamics across the entire hertz to mid-kilohertz regime that were correlated with reduced CO(2) and O(2) permeabilities. Using the centerband-only detection of exchange (CODEX) sequence, the fraction of flipping phenylene rings was observed to decrease in these polymers, with the decrease in the semicrystalline sample consistent with the crystalline fraction.