AbstractProton spin‐lattice (T1) and spin‐spin (T2) relaxation times of isotactic polypropylene films with different thermal histories, treated with tetrachloroethylene, were measured with pulsed NMR. For the melt‐quenched sample T11(the longer T1) increases and T2a(the longest T2) decreases with increasing treatment temperature. The mass fraction of mobile region (Fa) decreases and the mass fraction of rigid region (Fc) increases with increasing treatment temperature. T11increases almost linearly with increasing Fc, indicating that the removal of the mobile amorphous region is mainly responsible for the change in T11.For the annealed sample T11also increases with increasing treatment temperature, although a slight decrease in T11for the low treatment temperature is observed. There is a large decrease in T2awhen the treatment temperature is raised, suggesting that a comparatively selective extraction occurs for the annealed s