TiC and TiB2 particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB2 and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC: and TiB2 particle: additions are greater than those for Al2O3 and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB2 systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB2-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.