The melt viscosity of polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) blends relative to PC is significantly lower, even lower than that of pure ABS in some compositions. Annealing above the Tg of PC coalesces and coarsens phase structure in core and skin regions. Increase in the molecular weight of PC in PC/ABS blends results in low-temperature fracture toughness improvement but suffers from the disadvantage of higher melt viscosity. The selection of PC in PC/ABS blends must be a compromise between the toughness advantages of higher PC molecular weight and the disadvantage of higher melt viscosity. © 1993 John Wiley & Sons, Inc.