Morphology formation and deformation behaviour of styrene/butadiene based thermoplastic elastomers containing 65 vol.-% polystyrene was investigated from room temperature down to the temperature close to the glass transition of the soft phase. The techniques used were dynamic mechanical analysis (DIVA), uniaxial tensile testing and different microscopic methods. Two different transitions were observed at lower temperatures: elastomeric to ductile thermoplastic behaviour, and ductile thermoplastic to brittle behaviour. SEM investigations revealed that these transitions were associated with a change in deformation mechanisms.