The effect of vacuum annealing on the thermal stability and phase transition of Ti3SiC2 has been investigated by X-ray diffraction (XRD), neutron diffraction, synchrotron radiation diffraction, and secondary ion mass spectroscopy (SIMS). In the presence of vacuum or a controlled atmosphere of low oxygen partial pressure, Ti3SiC2 undergoes a surface dissociation to form nonstoichiometric TiC and/or Ti5Si3Cx that commences at similar to 1200 degrees C and becomes very pronounced at >= 1500 degrees C. Composition depth profiling at the near surface of vacuum-annealed Ti3SiC2 by XRD and SIMS revealed a distinct gradation in the phase distribution of TiC and Ti5Si3Cx with depth.