In this paper we discuss the formation of the ternary compound Ti5Sn2Ga (tau) with the D8(m) structure. Phase equilibria with participation of the tau-phase were studied experimentally, and partial solidus projection and isothermal section at 1300 degrees C were plotted. At the solidus temperatures the ternary compound is in equilibria with two continuous solid solutions: (1) between Ti2Sn and Ti2Ga (named Ti-2(Sn, Ga)) and (2) between Ti5Sn3 and Ti5Ga4 (named Ti-5(Sn, Ga)(3-4)). At 1300 degrees C Ti2Sn and Ti2Ga do not form continuous solid solution, and the ternary compound participates in equilibria with Ti-5(Sn, Ga)(3-4), and Ti2Sn, Ti2Ga and Ti3Sn based phases. The formation energy of the D8(m)-Ti5Sn2Ga compound is calculated by using electronic density-functional theory (DFT) using pseudopotentials constructed by the projector augmented waves (PAW) method in the generalized gradient (GGA) approximation for the exchange and correlation energy. The calculated lattice parameters are in good agreement with the experimental values. According to the experiments and the ab initio calculations, the D8(m)-Ti5Sn2Ga compound is stable and possesses a domain of off-stoichiometry.