Blends of polycarbonate (PC) with ABS and polycarbonate with maleic anhydride grafted ABS (MABS) were prepared by melt blending, using a single-screw extruder with special mixing head over the whole range of compositions. The rheological behaviour was determined using a capillary rheometer. The melt viscosity-composition curves for PC/ABS blends showed negative deviation from the rule of mixture with minimum values over a wide range from 25 to 65 wt.% PC. These curves for PC/MABS also showed negative deviation, but the deviation is less than that for PC/ABS blends. Addition of ABS or MABS to PC improves the processibility. Morphological studies of these blends were done using a scanning electron microscope (SEM). Morphology of unmodified blends showed coarse dispersion, whereas that of the modified blends showed fine and lamellar dispersion.