Titanium aluminides containing high niobium additions emerged as an attractive alloy family for automotive and aero-engine applications. Their processing by near net shape casting is rather demanding, not only due to easy contamination but also due to the fact that microstructure formation during solidification and subsequent solid state transformations sensitively depends on alloy composition and the applied processing conditions. The sequence of phase formation during solidification of the ternary alloy Ti-45Al-8Nb was analyzed based on non-equilibrium thermodynamic calculations and solidification experiments. This alloy solidifies completely via the β(Ti) phase, because the nucleation undercooling for the α(Ti) phase is high enough to prevent its formation. Thermodynamic calculations and experiments are shown to converge at last, with only minor improvements being necessary to correctly describe the metastable eutectic reaction “Liquid → β(Ti) + γ-TiAl” that occurs at the end of the solidification path.