X-ray photoelectron spectroscopy (XPS) and medium energy ion scattering (MEIS) are used to determine chemical bonding and composition of ultra-thin films of mixed yttrium, silicon, and oxygen, formed by oxidation of metal on clean and pretreated silicon. XPS and MEIS analyses indicate that oxidation of yttrium on bare silicon results in a fully oxidized film with a significant fraction of Y-O-Si bonding. The mixed Y-O-Si structure results from the relatively rapid reaction between Y and the Si substrate to form yttrium silicide, followed by oxidation. The effect of various silicon pretreatments, including in situ oxidation and nitridation, on bulk and interface film composition are also examined. Transmission electron microscopy (TEM) of 40 Angstrom thick films indicates that the yttrium silicate films are amorphous with uniform contrast throughout the layer. MEIS shows evidence for a graded metal concentration in the dielectric near the silicon interface, with uniform oxygen concentration (consistent with full oxidation) throughout the film. Angle resolved XPS (ARXPS) shows no significant signal related to Si+4, as would be expected from a substantial SiO2 interface layer. Capacitance-voltage analysis demonstrates that a similar to 10 Angstrom equivalent oxide thickness can be achieved. The effects of ultra-thin silicon oxide, nitrided-oxide and nitrided silicon interfaces on silicon consumption during the oxidation of yttrium are investigated. When yttrium is deposited on a thin (similar to 10 Angstrom) SiO2 film and oxidized, a yttrium silicate film is formed with bonding and composition similar to films formed on bare silicon. However, when the interface is a thin nitride, the silicon consumption rate is significantly reduced, and the resulting film composition is closer to Y2O3. The consumption of the silicon substrate by metal is shown to occur during oxidation and during vacuum annealing of yttrium on silicon. The relatively rapid formation of metal-silicon bonds suggests that metal-silicon structures may also be important reactive intermediates in silicon/dielectric interface formation reactions during chemical vapor deposition. In addition to thermodynamic stability, understanding the relative rates of elementary reaction steps in film formation is critical to control composition and structure at the dielectric/Si interface.