Layered ternary carbides and nitrides (e.g., Ti 3 SiC 2 and Ti 2 AlN) have attracted much attention as candidate materials for high-temperature structural applications. The high-temperature stability of these compounds is crucial in this respect. The susceptibility of these compounds to thermal dissociation at elevated temperature in high vacuum has been studied using in situ neutron diffraction. Above 1300°C, Ti 3 SiC 2 and Ti 2 AlN readily decompose to TiC x and TiN x respectively mainly through the sublimation of silicon and aluminum, respectively. The apparent activation energies for the thermal dissociation were determined using Arrhenius's equation. The kinetics of phase decomposition of Ti 3 SiC 2 , Ti 3 AlC 2 , and Ti 2 AlN was also modeled using a modified Avrami equation. Examination of cross-sectional microstructures of decomposed samples conducted using SEM support the findings of the neutron diffraction that the decomposition proceeds via sublimation of Si or Al from grain surfaces. The characteristics of thermal stability and phase transition in Ti 3 SiC 2 , Ti 3 AlC 2 , and Ti 2 AlN are discussed in this chapter.