AbstractThe glass transition behavior of two sets of ABA poly(styrene‐b‐isoprene) block copolymers was examined by differential scanning calorimetry. In one series, the triblock copolymers had different total molecular weights and the same (30 wt %) polyisoprene content, in the other, the molecular weight was constant (30,000 g/mol) and the elastomer content was the variable. For all triblock copolymers studied, the data show an inward shift for the glass transition temperaturesTgof the corresponding homopolymers. This shift increases for the rigid‐phaseTgas the polystyrene block length decreases. Depending on their molecular characteristics, two, three, or only oneTgwere found. The thirdTgwas interpreted in terms of the existence of an interphase. Some of these conclusions could be confirmed by transmission electron micro