Phase equilibria in the samarium-boron system were investigated by X-ray and metallographic techniques. Melting behavior and temperatures, or limits for these temperatures, were determined and used in constructing a temperaturecomposition phase diagram. Four intermediate binary phases were found: SmB, (x=2.0f0.6), SmB,, SmB,, and SmBoo. The tetra-and hexaborides were well known, and phases isomorphous with SmB,, have been reported, but the SmB, phase gives an X-ray diffraction pattern different from that of any known boride. Lattice constants were calculated for the 3 phases with known structures; the composition dependence of the values for the tetra-and hexaborides indicates that both phases have measurable composition ranges. Measured Vickers' microhardness values increase markedly for phases with increasing boron content. The SmB, phase is very soft compared to most boride phases.