The isothermal section of the phase diagram of the ternary system Hf-Ga-Si was constructed in the whole concentration range at 600ºC, using X-ray powder and single-crystal diffraction. Continuous solid solutions Hf 5 Ga 3-x Si x (x = 0-3) and Hf 2 Ga 1-x Si x (x = 0-1) are formed between the isostructural binary compounds Hf 5 Ga 3 and Hf 5 Si 3 (structure type Mn 5 Si 3 ) and Hf 2 Ga and Hf 2 Si (structure type CuAl 2 ). The other binary compounds of the systems Hf-Ga and Hf-Si do not dissolve noticeable amounts of the third component. One ternary compound, HfGa 0.33 Si 0.67 , is formed in the system (structure type TlI, Pearson symbol oS8, space group Cmcm, a = 3.7338(7), b = 9.889(2), c = 3.7441(7) Å). No tendency towards ordering of the Ga and Si atoms was observed in the structures of the ternary phases.