The phase diagram of the MoCo system between 800 and 1300°C has been investigated by microscopy, X-ray diffraction and electron-probe micro-analysis, using diffusion couples consisting of the pure metals or their alloys. The μ (Mo6Co7) phase has a constant homogeneity range of 7 at.% (51.5-58.5 at.% Co). The θ phase (discovered by Quinn and Hume-Rothery2) is stable between 1018 and 1200°C. Its Mo-rich boundary is found at 81 at.% Co. The Co-rich boundary shifts from 83 at.% Co at 1050°C to 81 at.% Co at 1200°C. The K phase found up to 1000°C has a homogeneity range of 75-76.5 at.% Co. The σ phase is found at and above 1000°C at a composition of about 36 at.% Co. The σ layers were too thin for an accurate determination of the phase boundary concentrations. Co can dissolve 19.5 at.% Mo, and Mo only 1.5 at.% Co at 1300°C.