Two boron rich phases were successfully synthesized by borothermal reduction of Yb oxide under vacuum. For the new boron-poorer phase, the single phase was established at around [B]/[Yb]=43.3 at 1500 C (Pbam space group; YB50-type; a=16.5811(5) Å, b=17.5950(5) Å, c=9.4647(3) Å; powder X-ray diffraction; Rietveld refinement). The crystal structure of the boron-richer phase ([B]/[Yb]=56.0) has been elucidated by single crystal X-ray diffraction ( space group; YB66-type; a=23.3587(6) Å). Powder X-ray diffraction data of the alloy YbB70 annealed at 1825 C yielded, along with the YB66-type compound (a=23.3691(2) Å), -rh B as a secondary phase ( space group, a=10.9298(3) Å, c=23.875(1) Å), for which the solubility of Yb was found to be below 1 at.%. Both YbB43.3 and YbB56.0 feature complicated boron atom frameworks which exhibit shorter B-B separations both within and between boron clusters as compared to those observed for prototype structures.