Composites reinforced with plant fibres show inelastic deformation under tensile loads. This property can be exploited in vibration-damping applications. Inelastic deformation was characterised for composites made from unsaturated polyester resin reinforced with unidirectional flax yarns. The primary yield point was found at a strain of 0.17%, with additional minor yielding events to a strain of 1.0% and then linear deformation to failure at a strain of 1.9%. Inelastic deformation accounted for approximately half of the total deformation across the final half of the stress-strain curve. Energy loss was greatest in the first load-unload cycle, with little change beyond the second cycle. The ratio of lost energy to stored energy settled to values between 0.23 and 0.40 for load-unload cycles peaking at between 15 and 75% of predicted maximum load. Failure stress remained unchanged after 2 x 10(4) load-unload cycles to approximately 50% of the breaking load. Results were consistent with kink bands straightening when fibres were first loaded, causing partial debonding at the fibre-matrix interface. Energy loss in subsequent cycles was attributed to friction between microfibrils within the kink bands as they twisted and untwisted.