This paper describes the effects of structures on the glass transition of glassy polymers blended with a semi-crystalline polymer. Immiscible blends of PS/PP and PS/HDPE were prepared from commercially available polymers using melt processing and extrusion without additives. The weight fractions of the components were varied from 0 to 1. SEM analysis of the blends showed a range of morphologies over the composition range from small inclusions at low volume concentrations through intertwined co-continuous structures at specific intermediate compositions, and a reversal of this configuration at high volume fractions. The glass transition of the glassy polymer was measured with differential scanning calorimetry using the sensitive and high resolution modulated DSC method. A systematic change in glass transition of glassy polymers is observed as a function of composition in various immiscible polymer blends. Results show that the glass transition of polystyrene increases with a reduction in volume fraction, by approximately 5.4°C in polypropylene and 6.5°C in polyethylene. Probable models which might explain this effect are mentioned.