The mechanical toughness of poly(phenylene oxide) (PPO) thin films was found to degrade remarkably from physical aging and the fracture mode transformed from ductile shear into a brittle type that involved no stable deformation zone growth. In the beginning, the fresh PPO films deformed almost uniformly upon stretching up to more than 20% strain. However, when the film was isothermally aged, diffuse shear deformation zones grew in response to the external deformation. The deformation zones could sustain extremely large strain (> 20% elongation) without cracking, but their breadth narrowed and boundaries sharpened as the aging time increased. As the aging time further increased, the diffuse zones were gradually replaced by the sharp straight shear zones that nucleated at a much lower strain and could break down during the course of deformation. These deformation zones had a generally smooth interior surface but sometimes showed no fibrillated microstructure and their growth followed a micro-necking mechanism. Finally, at the very long aging times, the PPO films cracked catastrophically at small strains around 1% elongation without the evidence of stable deformation zones. Increasing the aging temperature sped up this degradation process. A separate experiment using FTi.r. to examine the films cast from deuterated toluene indicated that the degradation was not due to the effect of solvent. The degradation, possibly due to the enthalpic relaxation during aging, however, requires further study to reveal its origin.