Three poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) thermoplastic elastomers (TPEs) are studied mechanically and compared to cross-linked natural rubber. It is observed that subtle alterations in the mid-block of the TPEs affect their mechanical properties significantly. The stress relaxation at room temperature is significantly altered indicating a reduced flow ill systems where the ratio of ethylene to butylene segments in the mid-block is greater than one. The cyclic behavior of these systems also shows significant hysteresis. Differential scanning calorimetry suggests that these TPEs crystallize Lit low temperatures, similar to the observed behavior ill cross-linked natural rubber. Results of internal energy changes from deformation calorimetry Provide evidence for strain-induced crystallization occurring in certain SEBS systems, similar to the irternal energy charges observed for cross-linked natural rubber. Simultaneous WAXD/SAXD measurements,on deformed SEBS samples highlight deformation at the nanometer and the Molecular length Scales. In situ WAXD at different strains further reinforces the evidence for formation of strain-induced crystallites in the selected systems. Strain-induced crystallization occurring ill certain TPEs provides it mechanism for reduction of flow at high strains and accounts for the retention of their highly elastic behavior. (c) 2005 Elsevier Ltd. All rights reserved.