Diffusion of helium, oxygen, nitrogen, argon, carbon dioxide, and methane in glassy and rubbery polythylene terephthalate has been studied in the range 25° to 130°C. Despite the abnormal solution behavior of these gases in the glassy polymer, the diffusion process is evidently normal and Fickian: Correlation of solubility constants for oxygen and nitrogen, obtained by the time‐lag method, with data obtained by a static sorption method, indicates that glassy crystalline polyethylene terephthalate may be considered an isotropic diffusion medium. Diffusion is impeded purely geometrically by the presence of the crystallites, and the impedance factor is equal to the reciprocal of the amorphous volume fraction. In the rubbery crystalline state of the polymer, diffusion is Fickian and apparent activation energies for diffusion are larger than those in the glassy crystalline polymer. A model for diffusion in the glassy amorphous and crystalline polymers is proposed, assuming that the driving force for diffusion is the concentration gradient of dissolved molecules which are assumed to be in local equilibrium with molecules in the ``holes.'' The model predicts that actual diffusion constants and activation energies are larger than those experimentally measured. Thus, part of the observed difference between values ofEDain the glassy and rubbery states may be reconciled. Correlations ofDandEDawith the square of gas molecular diameters are obtained in both the glassy and rubbery states. In the latter plots, the helium data fall above the correlations, showing evidence of partially non‐activated diffusion in both states of the polymer.