Secondary ion mass spectrometry (SIMS), Rutherford backscattering spectroscopy (RES) and X-ray photoelectron spectroscopy (XPS) are used to investigate Ti silicide formation mechanisms on a series of Ti on Si thin-films annealed in ultrahigh vacuum (UHV) at different temperatures and durations. The competition between oxygen diffusion and the silicide formation reaction (the so-called ’snowplough’ effect) is observed directly, as well as a Ti-Si-O layer. The results from these controlled experiments are compared with those from Ti-silicide films formed under rapid thermal annealing (RTA) conditions in a production furnace, with and without a TiW barrier layer. The TiW layer is shown to act as an effective barrier to silicon and oxygen out-diffusion, as well as the incorporation of ambient gases.