Methods are needed for indirectly assessing the physical state of historic textiles and further assessing the risk of display. Infrared spectroscopy offers a microsampling technique that can probe both the microstructure and chemistry of materials. A cellulose crystallinity index can be derived for plant fibres. Additional details were extracted from polarised spectra, with the coupled data allowing an estimate of the oriented fraction of the microfibrillar component. Applying this protocol to linen fibres suggested a 30 percent amorphous and 70 percent crystalline cellulose content, with 70 percent of the latter being well oriented. The relative changes in fractional oriented crystallinity were found to differ dramatically for a modern and a 16th-century linen, following accelerated ageing. However, there was no direct correlation with the breaking strain, and the change in mechanical behaviour could not be predicted with certainty on the basis of the oriented crystallinity parameter alone.