The relationships between distortion of the crystal lattice with deformation and fibre mechanical properties are complex. Micro-focus synchrotron diffraction is an ideal tool for probing such relationships. The investigation of fibres with different processing histories enables the influence of processing to be assessed. Longitudinal and transverse crystal strain is examined for three different varieties of PBO fibre (AS, HM and HM+). Large variations are found to exist between the different fibre types, although they share the same chemical structure. Crystal modulus calculated from longitudinal crystal strain and stress is found to be lowest in the fibre with the highest tensile modulus (HM+). Deformation increases the a-axis dimension of the unit cell for PBO AS and HM whilst decreasing both the a- and b-dimensions for HM+. These effects have been attributed to molecular chain rotation. It is also observed that the unit cell angle increases with deformation and the rate of change in unit cell volume is consistent between all fibre types, regardless of processing history. (C) 2004 Published by Elsevier Ltd.