Solid state phase equilibria in the ternary Fe-Ga-As diagram were determined at 600 °C using powder X-ray diffraction as experimental techniques. Very limited solid solubilities were measured in the binary constituent Fe-Ga and Fe-As compounds. In the Fe-rich part of the diagram, a ternary phase was evidenced which corresponds in fact to a solid solution into which Ga and As substitute one another on the same sublattice. This phase, which can be expressed by the general formula Fe₃Ga_2−xAs_x (0.20 ≤ x ≤ 1.125), crystallizes in hexagonal symmetry; it is structurally derived from the NiAs type-structure (B8₁) and can be considered, owing to its c/a ratio (1.23 ≤ c/a ≤ 1.28) close to $\sqrt{3}/\sqrt{2}$, as hexagonal-pseudocubic. The original feature of this experimental diagram is the occurrence of a tie-line between this ternary phase and the semiconductor GaAs which differs significantly from the theoretical diagram, estimated from simplified calculations proposed by Schmid-Fetzer. In particular, it is the first time that such a ternary phase M_x(Ga, As)_y (M = transition metal) is found to be in thermodynamic equilibrium with GaAs. This fact suggests that the solid state interdiffusions which occur during the annealing of a Fe/GaAs heterostructure could lead to a ferromagnetic, epitaxial and stable Fe₃GaAs/GaAs contact.