The microstructure of Al-Al2O3 composites made by reactive penetration of Al (or Al alloy) into ceramic (mullite or kaolin) preforms has been investigated using transmission electron microscopy (TEM). The Al-Al2O3 composites were found to contain a mutually-interconnected network of Al and Al2O3 NO crystallographic orientation was observed between the Al and Al2O3 phase. Impurities and pores in the ceramic preforms were found to have a strong effect on the microstructure of the composites. The impurities resulted in formation of small particles in the Al2O3 grains of Al-Al2O3 composites, whereas the porosity yielded a varied ratio of Al to Al2O3 in the composites. The growth rate of the Al-Al2O3 composites was found to depend on the microstructure and composition of the ceramic preforms as well as the composition of the reactive metals. Pure aluminium penetrated into a dense mullite faster than into a porous mullite at temperatures below 1200 degrees C. Addition of Mg to Al reduced the growth rate, whereas a continuous phase of amorphous SiO2 in the ceramic preforms increased the growth rate.