Molecular deformation during the mechanical straining of single filaments in a high-tenacity poly(ethylene terephthalate) (PET) technical yam has been investigated using Raman spectroscopy. A significant shift of over 2.5 cm(-1) in the peak position of the 1616 cm(-1) phenylene ring stretching Raman band was found during deformation up to fibre fracture, and this has been related to chain stretching of the polymer molecules in the fibres during macroscopic deformation. It is found that the change in the position of the Raman band is more closely related to stress than to strain, and the behaviour has been shown to be similar to that of deformed PET films and of high-performance fibres such as aromatic polyamides (aramids).