Unreacted PC/PBT blends have been obtained by melt blending in all the composition ranges. The followed processing method produced partially miscibilized and crystalline blends. The plot of the density of the amorphous part of the blends against its composition indicates that this parameter is more important than the crystallinity level in determining the mechanical properties of the blends. The blends exhibited an overall improvement of mechanical properties with a combination of behaviors that are close to linearity and are synergistic. © 1993 John Wiley & Sons, Inc.