The compressibility and phase stability of Y bB 2 are investigated under high pressure using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The bulk modules of high purity Y bB 2 is obtained as 182GPa using the Birch-Murnaghan equation of state. The patterns measured up to 20GPa and the pressure dependence of normalized lattice parameters, a/a 0 and c/c 0, reveal that the compressibility of Y bB 2 is low and fairly isotropic, and this material can be classified as a hard material. X-ray photoemission studies demonstrate that Yb in Y bB 2 has a mostly trivalent valence state at room temperature. Moreover, sample preparation details provide a new insight into the high purity synthesis of Y bB 2 at ambient pressure and moderate temperatures. The presented structural and compressibility results are in agreement with the available theoretical and experimental data on binary rare-earth borides and can serve as a reliable reference for future studies.