Raman spectroscopy has been used to investigate molecular deformation in a number of high-performance gel-spun polyethylene (PE) fibres. Well-defined Raman spectra can be obtained for the fibres and the study has concentrated upon the symmetric C-C stretching mode (1128 cm(-1)). During mechanical deformation, the Raman spectra show the existence of a bimodal molecular stress distribution in the crystalline phase resulting in splitting of the Raman band. The changes in the Raman band peak position and area with strain for different modes of deformation, including stress relaxation and creep, have indicated the molecular response of the material to stress is highly complicated. This information, however, is particularly useful in analysing the molecular deformation both quantitatively and qualitatively and it is shown that the Young’s modulus of the fibres is related to both the relative areas of the two Raman bands and their rate of shift per unit strain.