The phase transformations under elevated pressure have been followed in situ by X-ray and Raman spectroscopy for ultrahigh-molecular-weight polyethylene (UHMW-PE) reactor powder. Using the in situ X-ray as a reference for the Raman work, it has been shown that Raman spectroscopy is a convenient method to follow phase transformations in polyethylene under pressure. The distinction has been made between different solid phases in polyethylene (monoclinic, orthorhombic, and hexagonal phases) and the melt by studying the conformational changes occurring within the skeletal mode (1000-1200 cm-1) upon the phase transformations. The crystal field splitting (1415/1440 cm-1) in the -CH2- bending mode of the Raman spectrum of polyethylene, related to the orthorhombic unit cell, was confirmed to be sensitive to transformations between different crystal structures. By following the changes occurring in the methylene bending mode together with the changes within the C-C skeletal mode, it has been shown that the crystalline phases and the melt can be mutually distinguished.