To investigate the effect of a flame retardant on PC/ABS alloy systems, a reactive-type brominated epoxy resin was used as a flame retardant, and mixed with PC/ABS at various amount using a twin-screw extruder. The rheological, morphological, mechanical, and thermal properties were investigated as a function of acrylonitrile (AN) content in ABS. The shear viscosity of the PC/ABS blend increased as the AN content in ABS increased. Temperature drastically affected the shear viscosity in the ABS matrix, but not as much in the PC matrix. A blend of PC 50% containing AN 22% is suitable to process because its shear viscosity is low in the high shear rate region and its mechanical properties drastically increase in the beginning of the PC matrix. In addition, gradual increases of heat distortion temperature were observed in the entire range of the PC content. (C) 2000 John Wiley & Sons, Inc.