Modulated differential scanning calorimetry (MDSC) was used to study the glass-transition relaxation behavior in blends of poly(butylene terephthalate) (PET), a semicrystalline polymer, with polycarbonate (PC), an amorphous polymer. Using a temperature-modulated differential scanning calorimeter (TM-DSC), a sinusoidal temperature oscillation was superimposed upon the underlying linear temperature ramp. The reversing, total, and non-reversing heat flow curves were then analyzed We examined the efficacy of modulated differential scanning calorimeter (MDSC) to extract glass transitions (T-g) when these were covered over by rapid cold crystallization occurring in the same temperature range.Blends were available in PET/PC compositions of 80/20 and 40/60, and with high and low molecular weight, M-w, designated 'H' or 'L', respectively. Samples of very low initial crystallinity were prepared by rapid quenching from the melt. These samples crystallized immediately during the MDSC scan producing complex exothermic peaks when the scanning temperature increased over T-g. All blends exhibited a lower glass transition assigned to the PET-rich phase. The upper glass transition, assigned to the PC-rich phase, was never observed in 80H/20L, 80L/20L, or 40L/60L. This suggests that PC-L has better miscibility with amorphous PET, while producing a very broad, indistinct grass transition in the PC-rich phase. The Fox equation was used to determine the mass fraction composition of the two phases, and confirms that better miscibility is achieved when low molecular weight components are blended.Higher crystalline blends were prepared by melt crystallization. The size of the glass-transition step was greatly reduced in the melt crystallized blends compared to the quenched blends. Nonetheless, MDSC was used successfully to observe dual glass transitions at intermediate temperatures between the T(g)s of the homopolymers for all melt crystallized blends, except 80L/20L. Analysis of the lower T-g indicates better amorphous phase miscibility in blends with PBT-L and PC-L; analysis of the upper T-g indicates better amorphous phase miscibility in blends with PBT-L.