Blends of an amorphous and a semi-crystalline polymer-polystyrene and polypropylene, respectively-were prepared by melt processing in an extruder at 220 degrees C. These polymers are known to be immiscible and the composite morphologies were characterized by electron microscopy and thermal analysis. Fine micron-scale morphologies, ranging from 0.5 to 20 microns were observed. Thermal analysis and dynamic mechanical analysis showed changes in both the polystyrene and polypropylene glass transition temperatures (T-g) over the composition range. The major effect was a sharp increase in polystyrene T-g with increasing polypropylene content in the blend. A T-g elevation of 5.5 degrees C was observed at 85% polypropylene. The polypropylene T-g also increases with increasing polypropylene content, starting at a depressed value in discrete polypropylene domain environments and approaching the bulk polypropylene value after the phase inversion is crossed. Qualitative structural models are proposed based on spatial and mechanical interactions between the components.