We report a new binary base-centered orthorhombic phase of Cu2-xS. The new phase was obtained by chemical epitaxy on the (001), (111)A and (111)B faces of GaAs. Chemical analysis showed a Cu to S ratio of slightly less than 2 : 1, along with no evidence for other cations which can potentially replace Cu. Cross-sectional and plan-view samples were studied by TEM along multiple zone axes of Cu2-xS films and several GaAs orientations. Electron diffraction and fast Fourier transform patterns were analyzed and compared to the simulated data of known Cu2-xS phases with no match. A match was found with a new, base-centered orthorhombic unit cell with lattice parameters of a ≈ b = 5.6 Å and c = 11.2 Å. Seven different variations confirmed the well-defined orientation relationship existing between the base-centered orthorhombic phase of Cu2-xS and the GaAs substrate.