Layered ternary carbides (Ti3SiC2 and Ti3AlC2) are candidate materials for high-temperature structural applications. Their susceptibility to thermal dissociation at elevated temperatures in high vacuum has been studied using in situ neutron diffraction. Above 1300 °C, Ti3SiC2 and Ti3AlC2 decompose to TiCx mainly through the sublimation of silicon and aluminum, respectively. The apparent activation energies for thermal dissociation were determined using Arrhenius’ equation. The kinetics of the phase decomposition of Ti3SiC2 and Ti3AlC2 were modeled using a modified Avrami equation. Examination of microstructures using SEM shows that decomposition proceeds via sublimation of aluminum from grain surfaces. The high-temperature stability of Ti3SiC2 is improved with reinforcements TiSi2 and TiC. The phase transition of Ti3SiC2 and Ti3AlC2 upon thermal dissociation are also discussed.