The effect of additives on glass transition behavior in melt processed blends of polystyrene (PS) and polypropylene (PP) was studied. Blends of additive-free polystyrene and additive-free polypropylene revealed the known effect of the PS T-g increase in blend compositions where PP surrounds PS. Glass transition behavior in these blends was compared to blends prepared from additive-free PP and commercial grade PS, which contained lubricant additives. The thermal transitions of PS and PP were measured using modulated DSC. Although the behavior of low PS concentration blends was similar in both systems, the characteristics of the high PS blends differed substantially. These differences and the contrast in the PP T-g behaviors were attributed to the migration of additives from the PS phase across the immiscible interface into the PP phase. Similar T, variations were observed in blends of commercial grade PS and commercial grade PP. (C) 2007 Wiley Periodicals, Inc.