The depression of glass transition temperature, annealing-induced phase separation, and morphology were investigated in a binary solid solution of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD) in bisphenol A polycarbonate (BPAPC) and cyclohexyl polycarbonate (PCZ) with three different molecular weights. Differential scanning calorimetry, X-ray diffraction, microscopy, and FTIR were used to characterize these composites. Molecularly doped TPD in polymers significantly depressed the T-g of the host polymers. Although it could be expected that the TPD would disperse well in a flexible polymer and cause a substantial decrease in T-g, we find that the extent of depression is more pronounced with the rigid polymer. FTIR studies indicate that molecular level interaction exists between TPD and the polycarbonates, as shown by the frequency shifts in the aromatic region. Upon annealing, the phase separation and crystallization of TPD lead to a recovery of the T-g of the polymer, by 10-20degreesC, depending on the temperature and time of annealing. In the case of TPD/BPAPC composites, although the crystallinity of the phase-separated TPD was found to be close to 30%, the extent of recovery of the T-g was small. This is attributed to the plasticizing effect of the TPD crystals. A lower degree of crystallinity was observed in the case of TPD/PCZ, which is due to the lower molecular flexibility of the PCZ relative to BPAPC as well as the difference between the T-g of the polymer and the annealing temperature. The molecular weight and its dispersion has a significant effect on the recovery of the T-g in the case of PCZ, as well as the crystallinity of the phase-separated TPD.