The effects of the processing temperature on the morphology and mechanical properties at the weld line of 60/40 (w/w) polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) copolymer blends were investigated. The influences of the incorporation of poly(methyl methacrylate) (PMMA) as a compatibilizer and an increase in the viscosity of the dispersed ABS domain phase were also studied. The ABS domain was well dispersed in the region below the V notch, and a coarse morphology in the core region was observed. When tensile stress was applied perpendicularly to the weld line, the fracture propagated along the weak region behind the weld part; there, the domain phase coalescence was significant because of the poor compatibility between PC and styrene-acrylonitrile (SAN). Phase coalescence became severe, and so the mechanical strength of the welded specimen decreased with an increasing injection-molding temperature. The domain morphology became stable and the mechanical strength increased as the viscosity of the domain phase increased or some SAN was replaced with PMMA. That the morphology was well distributed behind the weld line and the mechanical properties of PC/ABS/PMMA blends were improved was attributed to the compatibilizing effect of PMMA. (C) 2004 Wiley Periodicals, Inc.