Yttria (Y{sub 2}O{sub 3}) is one of the candidate materials actively studied as a high dielectric constant replacement of amorphous silicon dioxide in downscaled microelectronic devices. An understanding of the physical properties of yttria epilayers on silicon is both a required prerequisite for applications and of fundamental interest. Using x-ray absorption spectroscopy at both the yttrium and oxygen K edges we study the local atomic and electronic structure in Y{sub 2}O{sub 3} epilayers deposited on Si(001). We find that above a thin near - interface layer the epilayers have a local atomic and electronic structure bearing a close similarity to that of bulk yttria; however, there is an increase in static disorder with decreasing thickness which can be correlated to an increase of the defectivity. The unoccupied electronic structure, as probed by near edge spectra at the cation and oxygen edges, is very similar to that of bulk yttria; we present and discuss simulations within the real - space full multiple scattering formalism which are quite successful in reproducing the experimental line shape. In the near - interface layer we detect the presence of silicon oxide, as reported for this and other systems previously; a new finding is that yttrium -silicon bonds are present in the interface layer, forming the precursors to the formation of yttrium silicide which develops into a long range ordered phase at higher deposition temperatures. We compare our observations with available theoretical predictions on the thermodynamic stability of yttria in comparison to silicon dioxide, yttrium silicide, and yttrium silicate; the predictions are not confirmed by experiment as far as the 2 …