The effect of pressure on phase transformations of ultradrawn ultra-high-molecular-weight polyethylene (UHMWPE) fibers was examined by in situ X-ray diffraction measurements with synchrotron radiation. The mesomorphic hexagonal phase was metastable even at a relatively low pressure of about 100 bar. The effect of pressure on the melting transitions was larger than in chain-folded structures. The existence and metastability of the hexagonal phase is the basis of compaction technology of UHMWPE fibers in composite materials. (C) 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 53-59, 2004.