Yyttrium ultra-thin films (coverage < 4 ML) and their interaction with oxygen on the Si(100)2 x 1 surface have been investigated in situ by Auger electron spectroscopy, low energy electron diffraction, thermal desorption spectroscopy, and relative work function measurements. The results show that yttrium develops in amorphous successive layers, interacting strongly with silicon at the interface, causing an early surface Y-Si intermixing and disruption of the silicon 2 x 1 symmetry. Yttrium increases drastically the oxygen sticking coefficient on silicon surface. It is surprising that even residual oxygen on the yttrium covered surface, induces a partial oxidation of silicon even at room temperature. By providing more oxygen, an Y-silicate compound (Y-O-Si) develops simultaneously with silica (SiO2) in a competitive way to each other. The silicate is thermally promoted by post-annealing process, probably due to the enhanced yttrium-silicon intermixing and/or inter-diffusion. Both of the silica and silicate are thermally stable compounds, which dissociate and desorb from the surface only at high temperatures ( > 1100 K). Desorbing species such as Y2Si and SiO were recorded and attributed to the cracking of silicate and silica respectively. A significant amount of yttrium always remains and/or diffuses into the silicon substrate, even after strong annealing. Such a procedure caused the appearance of a newly observed 3 x 1 phase corresponding to submonolayer yttrium coverage.