The constitution of cobalt-tin alloys containing 0 to 60 at. pct Sn has been investigated using optical metallography, X-ray diffraction, differential thermal analysis, and a solid electrolyte galvanic cell technique. The studies have established the solid solubility of tin in cobalt and the limits of the range of homogeneity at high temperatures of the filled-NiAs type gamma-phase. Temperatures of the invariant reactions liquid = alpha + gamma, liquid + gamma = CoSn, and liquid + CoSn = CoSn2 have been redetermined. Significant differences from previously reported data have been noted, and a revised equilibrium diagram is presented.