The effect of C addition on synthesis behavior, microstructure, reaction mechanism and high-temperature oxidation resistance of Ti-Si system was investigated. The Ti5Si3Cx-TiC composite was synthesized by introducing the C into Ti-Si system and using mechanically activated self-propagating high-temperature synthesis (MASHS). The addition of 1 mol C into 5Ti + 3Si reduced activation energy of reaction from 207.5 kJ/mol to 189 kJ/mol. The reaction in the Ti-Si-C system was initiated by the formation of TiC and followed by synthesis of an interstitial solid solution of Ti5Si3Cx, while reaction in Ti-Si was activated by solid-state diffusion of Si into Ti. The ignition temperature of the reaction was reduced from 1000 degrees C to 900 degrees C when the combustion temperature slightly increased as a result of the C addition into the system. The wave velocity of the reaction was also declined from 55 mm/s to 48 mm/s. The microstructural observations show that dissolution of the C in Ti5Si3 crystal structure restricted microcracks formation through coarse grains. Moreover, the oxidation resistance of titanium silicide was improved by the formation of the interstitial solid solution.