Raman and X-ray diffraction spectroscopy of nonstoichiometric phases in the yttria-rich side of the Y2O3-ZrO2 binary oxide system is presented. Samples with Y2O3 Content from 10 to 100 mol.% were prepared by melting powder mixtures in air using an arc-image furnace. The main peak in the X-ray diffraction spectra corresponds to the reflection (111) of the zirconia-yttria cubic solid solution and showed a continuous shifting with increasing zirconia content and also broadening in a wide compositional region. The widest broadening was found at around 30 mol.% Y2O3. The observed features can be attributed,to the superposition of different but close peaks corresponding to several solid solutions with different symmetry. However, a reliable analysis of the presence of different cubic phases cannot be performed by conventional X-ray spectroscopy. To my knowledge, the evolution of Raman spectra in this compositional region is presented for the first time. Clear changes are observed in the Raman spectra. The progressive reduction in the intensity and final disappearance of the main Raman peak characteristic of the sesquioxide-based phase is observed with increasing zirconia contents. A corresponding increase in the intensity of the Raman features characteristic of the defective fluorite-type structure of yttria-stabilized zirconia solid solution was also observed. Raman features corresponding to both phases were present in a wide interval of compositions, from small addition of only 5 mol.% ZrO2, but no variation in the Raman shift of the main peaks was observed, reflecting that the Y-O bonding in the yttria-rich phase was not significantly affected with zirconia addition. Broadening of the peak was ob served, and was attributed to the reduction of the coherent size of the yttria-based domains. The evolution of the spectra can be well explained using a two-phase model. Raman results indicate that substitution of yttria by zirconia in the lattice of the yttria-based sesquioxide domains is very limited.