High strain rate tensile impact properties of aliphatic polyketone terpolymers were investigated and related to the polymer chain structure. Aliphatic polyketones were used as a model system, by changing the termonomer content and type. Aliphatic polyketone is a perfectly alternating copolymer and the structure was changed with the addition of a few mol% of termonomer: propylene, hexylene and dodecene. Studied were the thermal properties with DSC and DMTA, tensile behaviour, notched tensile impact behaviour, notched Izod properties and the temperature development during deformation. The perfectly alternating copolymer had a melting point of 257 8C, a Tg at 15 8C, a high crystallinity (48%), a high yield stress (77 MPa) and yield strain (31%) but a relatively low fracture strain (85%) and an impact strength (notched Izod) of 13 kJ/m2. Increasing the propylene content to 6%, lowered the melting temperature to 224 8C, without changing the Tg. The modulus and yield stress were lowered but the impact strength improved. Increasing the length of the termonomer while keeping the Tm at 224 8C lowered the Tg, the modulus, the yield stress but strongly improved the impact resistance. The longer termonomers, with a lower yield stress, reduced the necking behaviour. The temperature increase in front of the notch was about 85 8C. By adding termonomers to aliphatic ketones, the notched impact behaviour improved significantly at the cost of modulus and yield stress.