TiN formation on TiSi2has been studied usinginsituRutherford backscattering spectroscopy and Auger electron spectroscopy. Two systems were investigated: (i) the reaction of Ti with nitrogen and (ii) the nitridation of TiSi2. When a Ti film on Si is annealed in N2or NH3, two reactions occur simultaneously: TiN forms at the surface and TiSi2at the interface. Oxygen, dissolved in the original Ti film, is expelled from the growing silicide and is piled up at the TiN/TiSi2interface. Around 600 °C, the complete conversion of the upper nitrogen‐containing layer into TiN is retarded by this oxygen and a TiNxO1−xlayer remains detectable between the TiN and TiSi2layers. At 750 °C, the TiN layer is formed very rapidly and further growth is blocked by the TiSi2layer that has developed underneath. Nitridation of TiSi2requires a temperature of at least 800 °C. Starting at the exposed surface, the silicide layer is gradually converted into TiN. Some of the Si released in this reaction segregates to the surface and forms islands. Most of the Si, however, segregates to the Si substrate where epitaxial regrowth occurs. Nitridation of TiSi2, therefore, results in rough layer structures, in contrast to nitridation of Ti on Si. The differences in layer morphology for these two processes are clearly evidenced by transmission electron microscopy cross sections.