Directed melt oxidation (DMOX) of pure aluminium has been used to produce Al/Al2O3 composites by growth into a particulate alumina filler in the absence of any dopants apart from a beta-Al2O3 impurity in the filler. The microstructural development and mechanisms of growth of these composites have been investigated. It is shown that the Al2O3 filler used in this work has both chemical and physical effects on the reaction process. The beta-Al2O3 impurity introduces sodium into the system; this increases the wettability of alumina (both filler and oxidation reaction product) by molten aluminium, and initiates DMOX reactions. In addition, the filler particle size has an effect on the directed oxidation reaction. If the particle size is too fine, no oxidation growth takes place. Filler particles limit the ingress of oxygen through the reaction front so that AlN instead of Al2O3 may be formed in regions behind the main reaction front. Although such AlN production is seen when magnesium is used as a dopant to initiate DMOX reactions in the Al/Al2O3 system, it is more marked with sodium, because the latter has a greater effect on the wettability of alumina by aluminium.