Polyethylene naphthalate (PEN) fibres possess a higher initial stiffness than that of polyethylene terephthalate (PET) fibres and this makes them an attractive competitor for use in mooring ropes and other applications for which a low compliance would be an advantage. The two types of fibres have been characterised and compared in tension, creep and fatigue and found to behave in very similar ways. Failure of both fibres results in similar fracture morphologies although under high cyclic loading a new failure process has been observed for the PEN fibres which combines step by step crack propagation and final failure normal to the fibre axis. In the light of this observation, similar fracture behaviour has also been identified in PET fibres and which, until now had been overlooked. The loading criteria for fatigue failure are similar for both fibres and it has been shown that, for a given maximum cyclic load, lifetime is raised if the minimum cyclic load is increased. (c) 2006 Springer Science + Business Media, Inc.