The diffusion of pentane and propane in TFE/PDD copolymer was studied using pulse field gradient NMR as a function of time after sample preparation. Pentane and propane display tortuous diffusion behavior in TFE/PDD copolymer (65% PDD) which means the apparent diffusion constant of these molecules slows as a function of the time, milliseconds to seconds, over which diffusion is observed in the pulse field gradient NMR experiment. High-permeability polymers are known to have changes in permeability as a result of aging and conditioning. These changes occur over a period of hours, days, and weeks,so the determination of apparent diffusion constants was repeated on the same sample.,over this time scale. Pentane or propane was added to a freshly cast film of the copolymer and then sealed in an NMR tube. The first measurement of apparent diffusion constants was made within 24 h and then repeated a few weeks later and then a year later. Diffusion became more rapid after a few weeks which was ascribed to conditioning by the penetrant. More rapid diffusion at short times or small length scales indicates the introduction of high free volume regions, and the more rapid diffusion at longer lengths scales indicates improved connectivity between these regions. After a year, diffusion at short time scales corresponding to short length scales slowed. This was interpreted as resulting from the densification of smaller high free volume regions. Over longer length scales diffusion remained relatively constant which was interpreted as little change in the connectivity of high free volume regions. Diffusion as seen in the pulse field gradient experiment was compared for two copolymer compositions: 65% PDD and 87% PDD. Diffusion in 87% PDD copolymer is faster and no evidence of tortuosity is observed, both of which are consistent with a larger proportion of high free volume regions.