The Co-Ga binary system has been investigated by X-rays in conjunction with differential thermal analysis.It has been established that besides Ga and γ-Co which is the room temperature modification of pure cobalt, there are three phases in the entire system, α′,β and ψ. α′ is a superstructure phase based on a face-centred cubic fundamental lattice, and may be considered as a solid solution of α-Co which is the high temperature form of pure cobalt, stabilized at room temperature due to the addition of gallium. The homogeneous range of this phase at room temperature is 2.4-10.7 at % Ga. The β phase is formed by a peritectic reactio α+L ←→β at 1177℃, with a homogeneous range 36.3-59.8 at % Ga at room temperature. The structure of this phase is of the CsCl (B2) type. The lattice spacing changes with composition in such a way that it rises at first, reaches a maximum, and then falls off with the gallium content. This is similar to the β phase in the Al-Ni system. In the Co-rich side of the maximum, it appears as a substitutional type solid solution, while in the Ga-rich side, it belongs to the defect type, the addition of Ga atoms being equivalent to the dropping off of some Co atoms from the normal lattice. The homogeneous range of the ψ phase is extremely narrow, existing only at the near neighbourhood of the stoichiometric composition 75 at % Ga. This phase belongs to the tetragonal system with space group D4h14-P42/mnm. Bach unit cell contains four formula weights CoGa3, and the lattice spacings at 75.4 at % Ga and 20℃ are α=6.2365?, c=6.4347?, c/a = 1.0318. It is formed peritectically (α + L←→ψ)at 833℃.Neither Ga in γ-Co nor Co in Ga shows any detectable solid solubility. There exists an eutectic horizontal from CoGa3 to Ga at 27℃, with the eutectic point very close to pure gallium.