In situ Al/TiB2 metal matrix composites (MMCs) have been successfully produced by Salt-Metal reactions. This is a novel low-cost reactive approach, which involves adding Ti and B bearing salts to molten Al. The reactions between the salts lead to the formation of the reinforcing TiB2 particles in the Al matrix. The in situ formed TiB2 particles are very fine (below similar to 1 mu m in size). Strings and clusters of particle agglomerates are distinct microstructural features of all the composites with pure Al as the matrix. The effects of processing parameters on the kinetics of TiB2 formation and on the final microstructures are studied in detail. Besides, efforts are made to improve the distribution of TiB2 particles in the Al matrix by means of chemical additions; it is found that a homogeneous distribution is obtained by using a eutectic Al-Si alloy as the matrix material.