A range of SiC-B4C composites have been prepared by pressureless sintering, using different proportions of two sizes of B4C; 7 and 70 mu m. The interfacial area between the B4C and SiC has been quantified and is shown to have a significant effect on both densification and the resultant microstructure of the composites. SiC/B4C interfaces typically hinder densification. SiC/B4C interfacial area is also shown to be related to grain growth and polytype distribution in the SiC. With more SiC/B4C interfacial area, grain growth in the SiC is restricted and less of the SiC transforms from the starting 6H polytype to the 4H one. It is therefore suggested that it may be possible to use SiC/B4C interfacial area as a means by which to engineering the microstructure.