Structure-property relationships in a number of different aromatic high-modulus fibres have been examined in detail. Fibres studied include Kevlar, Twaron, Technora and PBO and it has been shown using Raman spectroscopy that these fibres exhibit similar molecular deformation processes. The stress-induced shift of the 1610 cm(-1) Raman band, assigned to a stretching mode of the p-phenylene ring, is found to be approximately -4.0 cm(-1)/GPa for all of the fibres, independent of their chemical structures and morphologies. A similar rate of shift is found for the same band in poly(ethylene terephthalate) fibres. This implies that the molecular deformation processes in these different fibres are similar even though the fibres have different chemical structures and morphologies, and have been processed differently. It is shown that the mechanical behaviour of the fibres is consistent with deformation following a uniform stress series aggregate model.