Data for the diffusion of ethane in polyethylene are presented together with the results of optical microscopy and of large‐ and small‐angle x‐ray diffraction for the same samples. The orientation of the lamella boundaries with respect to the concentration gradient of the diffusing molecules is important in determining the diffusion rate. In unoriented, as‐crystallized polymers, the diffusion constant varies inversely with the lamella thickness. The results support the concept of lamella boundary diffusion. Lamella perfection is also important. In both oriented and unoriented polymers, the solubility constant apparently varies inversely with the lamella thickness, suggesting absorption in the lamella boundaries.