This article presents an experimental study on the microscopic mechanisms associated with crack propagation in closed cell polymer foams. A brittle, slightly cross-linked polyvinyl chloride (PVC) foam of density 60 kg/m(3) and a ductile thermoplastic polyether sulfone (PES) foam of density 90 kg/m(3) were examined. The PVC and PES foams have similar cell size (a parts per thousand 0.7 mm) but the cell edges of the PES foam were much thicker than those in the PVC foam. Overall, it was observed that the elements of both foams fractured in an extensional mode. Crack propagation in the PVC foam was inter-cellular, where agglomerates of very small cells formed a region of weakness. Damaged cell walls were observed on both sides of the crack plane. For the PES foam, craze-like deformation bands were observed in the highly stretched region ahead of the blunted crack tip. Further ahead of the crack tip, highly stretched cells were observed. Fracture occurred predominantly through the center of the cells in the PES foam.