Metal matrix composites (MMCs) consisting of an Al matrix reinforced with initially added BN particles and newly formed AlN dispersoids were fabricated using the pressureless infiltration method. This method led to the incorporation of Mg and nitrogen into the Al matrix. In the retained BN dispersoids, the depletion of nitrogen and enrichment of Al and Mg occurred. The more Mg available, the more AlN formed in the MMCs. The oxidation resistance of the prepared MMCs at 773 and 823 K in air was not satisfactory, owing to the formation of MgO-rich scale. Mg, which was used as an infiltration enhancer precursor, oxidized preferentially, and thereby decreased the oxidation resistance of the prepared MMCs.