Increasing the Al content in γ-TiAl based alloys lowers density and improves the oxidation behavior. Although with the given chemical compositions lamellar microstructures of γ-TiAl and r-TiAl2 can be adjusted, the occurrence of metastable phases such as h-TiAl2 and Ti3Al5 impairs the formation of an equilibrium state and embrittles the alloy. The effects of additional alloying elements on the microstructural evolution of Al-rich TiAl alloys are mostly unknown and were investigated in the course of this work for Mo, Nb and B. Polycrystalline sample material with different compositions was manufactured by levitation melting and vacuum arc remelting. Several heat treatments were carried out to make a proper comparison to the binary alloy system. Samples were investigated via scanning electron microscopy. X-ray diffraction was performed on powders to identify the occurring phases. Electron micro probe analysis was executed to measure phase compositions and solubility limits. To investigate transition temperatures of stable as well as metastable phases differential thermal analysis and differential scanning calorimetry was performed. The as-cast state revealed process related defects and cracks and Mo has segregated in all specimens during cooldown, whereas Nb did not. Homogenization was possible via holding the samples at 1400°C for one hour followed by furnace cooling. Subsequent Heat treatments of 200 hours at 1000°C and 500 hours at 800°C followed by water quenching were performed. The addition of Mo and Nb impaired the microstructural evolution and lowered the amount of r-TiAl2 but it was still possible to produce lamellar structures of γ-TiAl and r-TiAl2 for 1 at.% Mo. None of the specimens reached an equilibrium state. The range of the γ-TiAl + r-TiAl2 double phase region decreased with an increasing Mo content as well as the stability of the embrittling Ti3Al5. Transition temperatures were measured and a quasibinary phase diagram was generated.