An investigation of the phase equilibria and phase transformation behavior in the Al-Ti-Ta system was conducted with an emphasis on developing alloys with stable microstructures and properties desirable for advanced aerospace applications. A series of ternary alloys containing 5 to 50 atomic percent Ta and varying amounts of Ti and Al were prepared by arc melting and characterized in the as-cast and heat treated (1450°C) conditions. Three stable two-phase regions (β-σ, β-α2. and Al2 Ta-η) were identified and characterized at 1450°C. A stable ternary B2 phase was identified near the composition 25Al-5OTi-25Ta1 and several binary phases were found to exhibit extensive solid solubilities. The results were used to construct a partial isothermal section at 1450°C for this system and to assess the complex phase transformations that occur upon cooling.