Blends of aliphatic polyketone terpolymer and a core-shell rubber (CSR) were melt processed with varying CSR concentration of 0– 40 wt%. The obtained morphology was of finely dispersed CSR particles in the polyketone matrix. The thermal properties of the matrix polymer remained unaffected by the addition of the CSR phase. The crystallinity remained constant at 35 wt% and the melting temperature was not changed. The tensile modulus and yield stress were decreased by the addition of the rubber phase to the aliphatic polyketone polymer. The deformation was strongly delocalised with increasing CSR content. The temperature development during fracture was also strongly reduced with increasing rubber concentration. The CSR phase was found to toughen the aliphatic polyketone matrix very effectively, the brittle to ductile transition temperature was lowered from 90 to 240 8C with the highest rubber concentration (40 wt%). Cavitation experiments revealed that the macroscopic cavitation strain remained constant with increasing rubber content. A study of the deformation zone below the fracture surface showed that voids were produced by cavitation of the rubber phase. The voids were strongly deformed by the plastic deformation of the matrix polymer. At high strain rates a relaxation layer was found below the fracture surface, where the voids were no longer present. This relaxation zone was found to be due to the adiabatic temperature rise of the material during fracture at high strain rates.