Polystyrene (PS) and bisphenol A polycarbonate(PC) molecular composites prepared by in situ polymerization techniques have been investigated by FT-IR, H-1 NMR, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA) and mechanical testing. The homogeneous PC-styrene monomer mixture (solution) polymerized in situ yielded a partially miscible composite. Partial miscibility was established by a comparison of the experimentally determined glass transition temperatures (T-g) with those estimated by an equation formulated from a completely miscible amorphous polymer system. The partial miscibility was also confirmed by images obtained from the SEM and optical micrographs, size exclusion chromatograms and dissolution behaviors in common organic solvents. SEM of the fracture surfaces revealed that the dispersed PC domains sized 1-2 mu m in diameter emerged from the continuous PS phase over time. Etching experiments confirmed that the PC domain contains subinclusions of PC/PS microdomains. Both the thermal and tensile properties of the composite were found to be enhanced than pure PS. The entropic gains by mixing the flexible component polymers and the possible formation of n-pi complex between carbonate-benzene mixture are proposed for the partial miscibility.