The paper presents the implementation of spectroscopic ellipsometry measurements in the vacuum-ultraviolet spectral range with use of synchrotron radiation as a light source. Current status of VUV ellipsometer and the principle of its operation is described. The procedure of measurements and ellipsometric data evaluation taking into account surface roughness of the measured specimen of optically uniaxial widebandgap single crystal is presented through the example of Sr61Ba39Nb2O6 ellipsometric characterization. Complex dielectric function ε(E) = ε1(E) + iε2(E) for Sr61Ba39Nb2O6 was obtained in the spectral photon energy range 2-25 eV for polarization direction along b and c crystallographic axes.