The deformation of three types of sulfone polymers-polysulfone (PSF), poly(ether sulfone) (PESF), and polyarylsulfone (PASF)-was performed with a mechanical testing system (MTS). The result and samples were studied using a transmission electron microscope (TEM), a dynamic mechanical analyzer (DMA), and a dielectric thermal analyzer (DETA). Stripes on TEM micrographs for small deformations increase with the fracture toughness (G(IC)). The activation energy (E(a)) of the alpha-transition was determined from DETA. The results show that both E(a) and TEM micrographs can be used to qualitatively estimate the order of G(Ic) of sulfone polymers. An energy model was proposed to explain this phenomenon.