A study has been conducted on the compressive deformation behaviour of thermally crosslinkable poly(p-1,2-dihydrocyclobuta phenylene terephthalamide) (PPXTA) fibres. The morphology of the failure zones was examined by scanning electron microscopy and dark-field transmission electron microscopy. On increasing the heat-treatment temperature from 260-400 degrees C, and therefore with increasing cross-link density, fewer kinks per unit length were displayed after compressive deformation. The kink specific energy was estimated to increase by a factor of 30, as determined by quantitative measurements of kink density at a given strain and of the critical strain to kink formation. Thus, the intermolecular cross-links still allowed deformation to proceed by kinking, but significantly raised the energy of kink formation. Finally, rupture zones were predominantly observed in axially compressed PPXTA fibres heat-treated at 440 degrees C. Compressive failure of the fibres changed from kink-dominated failure to brittle rupture with increased heat-treatment temperature, evidently as the result of cross-linking or of chain degradation. A dislocation model of the kink boundary developed by Vladimirov ef al. was analysed and critically compared with our data. The analysis of this theory with our experimental results suggested that the dramatic change in compressive behaviour with cross-linking was due to a transition from fine intermolecular shear to blocky interfibrillar shear.