Titanium aluminum carbide exhibits a unique combination of characteristics of both metals and ceramics coupled with an unusual combination of mechanical, electrical and thermal properties. In this paper, the effect of high-vacuum annealing on the phase stability and phase transition of Ti2AlC and Ti3AlC2 at up to 1550°C was studied using in-situ neutron diffraction. The decomposition of and Ti2AlC and Ti3AlC2 into TiC was observed from the change of relative phase abundances as a function of temperature. Apparent activation energies of phase decomposition of 85.68 and -71.9 kJ/mol were determined for Ti2AlC and Ti3AlC2 respectively. Nearsurface composition depth profiling using grazing-incidence synchrotron radiation diffraction has revealed a graded phase composition in vacuum-decomposed surfaces.