Transmission electron microscopy has been used to investigate the microdeformation behavior of thermally imidized thermoplastic pyromellitic dianhydride/1,1-bis(4-aminophenyl)-1- phenyl-2,2,2 trifluoroethylene (SFDA/PMDA) polyimide films with a T-g of similar to 440 degrees C, prepared by solution casting of a polyamic ester precursor. Failure of the films at room temperature was by unstable cracking at about 5% strain, accompanied by homogeneous shear deformation at the crack tips. As the temperature was raised to above 100 degrees C, zones of mixed shear and crazing were observed, and a stick-slip mode of cracking. Above about 300 degrees C shear was once again the dominant deformation mechanism and the films became fully ductile. In films containing porosity on a scale of a few nanometers, prepared by thermal degradation/imidization of a 3FDA/PMDA/poly alpha-methyl styrene graft copolymer, film failure at room temperature was also by unstable cracking, but a zone of multiple craze-like features was observed at crack tips, rather than a single shear deformation zone. The increase in extent of this zone of craze-like features as the temperature was raised was again associated with an increase in crack stability.