The location of the hexagonal crystalline phase of polyethylene in a pT-diagram, occurring at high pressure and high temperature, was determined from proton-NMR spin-lattice relaxation rates and found to be in good agreement with the results obtained with other techniques. Hexagonal and orthorhombic crystalline components as well as the amorphous part in a sample at high pressure can clearly be distinguished by deuteron NMR spectra. The changes in these components were followed as a function of temperature at various pressure values, showing the coexistence of orthorhombic and hexagonal crystalline materials during crystallization.