Poly(ethylene furanoate) (PEF), the furan-derived analogue to poly(ethylene terephthalate) (PET), can provide a fully biosourced alternative to PET with greatly improved barrier properties and attractive thermal and mechanical properties. The improved barrier for PEF compared to PET is unexpected due to the higher free volume of PEF vs PET. Segmental motions related to penetrant diffusion in both polyesters were studied using dynamic mechanical analysis, C-13-CP/MAS solid-state NMR variable contact-time experiments, and centerband-only detection of exchange (CODEX) measurements. Unlike the active phenyl ring-flipping mechanism in PET, furan ring-flipping is greatly suppressed, thereby reducing beta relaxation motions and diffusion in PEF due to the energy penalty associated with the nonlinear axis of ring rotation and ring polarity. Preliminary work also shows similar oxygen solubilities for PEF and PET, thereby proving that the drastic permeability reduction results from a decrease in diffusion coefficient caused by a hindrance in furan ring-flipping.