Alumina (Al2O3) test bars containing a small (5-10%) volume of titanium diboride (TiB2) or zirconium diboride (ZrB2) particles have been pressed and sintered (pressureless) in an argon atmosphere. The microstructure of the sintered bodies was characterized by X-ray diffraction and a range of microscopical techniques and shows that 3ppm (by volume) of oxygen present in the argon caused the boride particles in the surface regions of the test bars to oxidize during sintering, to a greater extent in the Al2O3-TiB2 composites. Mechanisms of oxidation are discussed. The boride particles retarded the densification of the composites, to a greater extent in the Al2O3-ZrB2 bodies. However, densification in the Al2O3-ZrB2 system was enhanced by sintering in an Ar-4% H2 atmosphere. The decrease in flexural strength due to the retardation of sintering has been overcome in both types of composite.