Titanium silicon carbide (Ti3SiC2) and Ti3SiC2-based composite powders were synthesized by isothermal treatment in an inert atmosphere as a function of initial compositions (mixtures). A high content of TiC was obtained in the final product when the initial mixtures contained free carbon. The use of TiC as a reagent was unsuccessful in obtaining Ti3SiC2. High Ti3SiC2 conversion was found for the initial mixtures containing SiC as the main source for silicon and carbon. An initial mixture with a large excess of silicon, 3Ti/1.5SiC/0.5C, was needed to obtain high-purity Ti3SiC2. A reaction mechanism, where Ti3SiC2 nucleates on Ti5Si3C crystals and grows by long-range diffusion of Ti and C, is proposed. The reaction mechanism was proposed to be based on silicon loss during the formation of Ti3SiC2.