Non-oxide ceramic nanostructured powders are synthesized through metastable transformation processing based on the Self-propagating High-temperature Synthesis (SHS) process followed by quenching. Binary systems like the investigated TiC–TiB2, when quenched from the liquid state give rise to metastable structures capable of being converted into a stable, fine-grained (nanocomposite) microstructure upon recrystallization by medium temperature treatments. A necessary condition is that the combustion temperature of the SHS reaction is higher than the eutectic temperature. A previous optimisation of the reaction stoichiometry was carried out to obtain SHS products with composition approximately equal to the eutectic (i.e. 67%mol TiC0,7 – 33%mol TiB2), according to the reaction: 6Ti + B4C + 1.8C → 4TiC0.7 + 2TiB2. In this work, different amounts of sodium borate (borax) were used in order to determine the optimum amount of additive to produce nanostructured TiC0.7–TiB2 composites. The morphological evolution of the powders after thermal treatment yielding re–crystallized structures demonstrates the metastability of the SHS–quench products. Therefore, the metastability process based on SHS– quench represents an extremely attractive route suitable for the achievement of nanocomposites.