Several structural and dynamic properties of unstretched and stretched ultrahigh molecular weight polyethylene (UHMWPE) films, as well as UHMWPE films doped with pyrene (Py/UHMWPE) and either irradiated with UV-vis (eV range) photons or bombarded with 4.5 MeV protons, have been investigated. X-ray diffraction patterns of the three-dimensional orthorhombic crystal structure of a uniaxially stretched UHMWPE film, prepared by the gel technique, have been completely indexed for the first time. A minor monoclinic crystalline component, whose importance depends on the draw ratio, is also present. Linear dichroism spectra of dopant pyrene molecules indicate that their long molecular axes are oriented to a smaller extent in highly stretched UHMWPE films than in polyethylene films of much lower crystallinity and draw ratios. When UHMWPE films were stretched, the heat of melting was unchanged, but the melting temperature increased. Stretched films bombarded with 4.5 MeV protons exhibited lower heats of melting and melting temperatures than unstretched bombarded films. When Py/UHMWPE films were bombarded with 4.5 MeV protons or irradiated with >300 nm photons under oxygen-free conditions, I-pyrenyl groups were attached covalently to polymer chains, yielding Py-UHMWPE. Diffusion of NN-dimethylaniline (DMA) in to and out of Py-UHMWPE films was followed in real time by fluorescence intensity changes. In unstretched films, at least two parallel diffusion pathways appear to operate. The activation energies for diffusion of DMA in unstretched Py-UHMWPE are larger than those in stretched Py-UHMWPE or in other types of polyethylene films. The results are discussed in terms of the structural differences between unstretched and stretched UHMWPE and other polyethylene types.