There is no consensus in the literature on the structure and composition of the ε phase of the W-B system, variously reported as WB2 and W2B5. We used ab initio calculations at two levels of theory to identify the stable crystal structure and stoichiometry of this phase. Among the sixteen structures investigated in the composition range of 67–71.4 at% B (WB2–W2B5), nine exhibited unfeasibly high formation energies; the remaining seven were dynamically stable (did not exhibit any soft phonon modes), and satisfied mechanical stability criteria. When including the thermal vibrational contribution to the free energy, all structures with the W2B5 composition lay above the convex hull, suggesting that this composition is metastable, while those with WB2 composition lay on the convex hull or within DFT precison of the convex hull. We found that four of the candidate structures exhibit negative vacancy formation energy, suggesting that the structures are unstable, or that they are naturally hypo-stoichiometric. Combining these results with a comparison of simulated and experimental x-ray and neutron diffraction patterns, we concluded that the ε phase is most likely a hypo-stochiometric W2B5−x compound with space group P63/mmc.