High-density polyethylene was studied by solid state H-1 NMR spectral fitting, relaxation, and spin diffusion methods. The results are compared with measurements made by Raman spectroscopy, solid state C-13 NMR, density, and DSC methods. A modified Goldman-Shen solid state H-1 NMR technique was used to estimate the thicknesses of microscopic domains of the polyethylene. Other information obtained is the mobility of polymer chains in the domains, the proton NMR spin diffusion rate constants, and the proton NMR spin-lattice relaxation times. Results indicate that the so-called interfacial phase of the polyethylene is significantly more dense than the amorphous phase and that transport of chains by the process called "chain diffusion" contributes to the mechanism of H-1 NMR spin-lattice relaxation.