While silicone rubber has a much lower shear viscosity than that of fluororubber, the viscosity of silicone rubber/fluororubber blends is closer to that of silicone rubber. All rubber compositions show pseudoplastic behavior. While the viscosity of a 50/50 silicone rubber/fluororubber blend increases on replacement of the silicone rubber by silicone rubber vulcanizate powder (SVP), the effect of fluororubber replacement by the corresponding fluororubber vulcanizate powder (FVP) on the shear viscosity is less pronounced. The difference in viscosity between SVP-replaced silicone rubber and FVP-replaced fluororubber becomes less prominent at higher shear rates. Fluororubber exhibits a higher extrudate die swell than that of silicone rubber. The die swell of the silicone rubber/fluororubber blends is higher than are the figures obtained by the additivity rule. Replacement of constituent rubbers in the blend by the corresponding vulcanizate powders causes an increase in the die swell. While the silicone rubber extrudate surface is smooth, the fluororubber extrudate shows melt fracture. The extrudate surfaces of the silicone rubber/fluororubber blends are similar to that of the silicone rubber extrudate. Replacement of constituent rubbers by the corresponding vulcanizate powders increases the roughness of the extrudate surface, which is more prominent in the case of silicone rubber replacement by SVP. SVP can replace 50% of silicone rubber and FVP can replace 75% of fluororubber in the 50/50 silicone rubber/fluororubber blend. (C) 2003 Wiley Periodicals, Inc.