Near infrared (NIR) Raman spectroscopy has been used to investigate the deformation behaviour of highly oriented fibres and tapes made from thermotropic aromatic copolyesters. It has been found that a number of Raman bands of thermotropic aromatic copolyesters show Linear shifts towards lower wavenumbers with stress and strain following tensile deformation. The strain and stress dependencies of the 1600 cm(-1) band due to stretching of the p-phenyl ring, have been studied in detail and discussed in relation to values reported for high-performance aromatic polymer fibres. The behaviour has been discussed in terms of the microstructure of the thermotropic aromatic copolyester materials compared with that of other highly-oriented aromatic polymer fibres.