Optical characterization of zirconia-yttria (YSZ) based single crystal compounds (9-32 mol% of yttria) has been carried out in the 5-60 eV energy range from reflection electron energy loss spectroscopy measurements. The stoichiometry and the chemical state of the elements at the surface of the samples were studied by x-ray photoemission spectroscopy. The procedure of analysis to obtain optical properties relies in a quantitative description of the electron energy losses. Inelastic mean free paths of 23, 13 and 5.8 Angstrom for electrons traveling in a reflected geometry in YSZ are obtained for electrons of 1500, 800 and 300 eV, respectively, independently from its yttria content. It has been found that there is a dependence of optical magnitudes on the Zr/Y ratio. Thus, the energy of the bulk plasmon excitations varies from 14.3 to 14.6 eV when the doping dose increases from 9 to 32 mol% of yttria. Surface plasmon excitations appear at 12.6 and 13.2 eV for the 9 and 32 mol% YSZ samples, respectively. Individual electronic transitions have been identified in the absorption coefficient. Besides, the analysis of the absorption coefficient suggests that for the increasing amount of yttria, the conduction band loses gradually the two peak structure observed in pure ZrO2 due to the crystal field splitting of the 4d orbitals of zirconium.