Transport properties of carbon dioxide in amorphous poly(ethylene furanoate) (PEF) were investigated using complementary pressure-decay sorption and permeation techniques. Detailed measurements for PEF at 35 degrees C indicate a significant, surprisingly large reduction in carbon dioxide permeability of 19x at 1 atm compared to poly(ethylene terephthalate) (PET), despite both an increase in free volume and carbon dioxide solubility of 1.6x for PEF vs PET. The solubility increase for PEF, which originates from greater interaction between carbon dioxide and the polar furan moiety, is offset by a substantial reduction in diffusivity of 31x compared to PET. Such diffusion reduction for PEF, which is 3x greater than the 9.7x reduction in oxygen diffusivity compared to PET, is thought to originate from a hindrance of polymer ring-flipping motions compared to PET. A possible mechanism for the surprising barrier improvement for carbon dioxide in PEF vs PET is explained herein along with a detailed comparison to oxygen and water transport.