Differential scanning calorimetry (DSC) does not allow for easy determination of the glass-transition temperature (T-g) of the polystyrene (PS) block in styrene-butadiene-styrene (SBS) block copolymers. Modulated DSC (MDSC), which deconvolutes the standard DSC signal into reversing and nonreversing signals, was used to determine the (T-g) of both the polybutadiene (PB) and PS blocks in SBS. The T-g of the PB block was sharp, at -92 degreesC, but that for the PS blocks was extremely broad, from -60 to 125 degreesC with a maximum at 68 degreesC because of blending with PB. PS blocks were found only to exist in a mixed PS-PB phase. This concurred with the results from dynamic mechanical analysis. Annealing did not allow for a segregation of the PS blocks into a pure phase, but allowed for the segregation of the mixed phase into two mixed phases, one that was PB-rich and the other that was PS-rich. It is concluded that three phases coexist in SBS: PB, PB-rich, and PS-rich phases. (C) 2004 Wiley Periodicals, Inc.