The advantage of titanaluminide compared to other high temperature materials is their lower density as well as their good mechanical properties. The addition of Niobium and Molybdenum leads to a better processing due to the beta-stabilizing effect. The mechanical properties are related to the phases alpha, beta and gamma and their volume fraction. That is the reason why a phase diagram is of great interest for the alloy processing. A fluctuation of the chemical composition can happen easily in production. Alloys with 41, 42, 43, 43.5, 44 and 45 at% Aluminium were investigated. Furthermore, an alloy with lower content of Molybdenum was produced to study the effect of the alloying element. The temperature of phase transformation was detected by means of a differential scanning calorimetry (DSC). A binary cut through the multiphase diagram was constructed by these results. To verify the phase diagram heat treatments were conducted. The microstructure was frozen by water quenching. With scanning electron microscopy (SEM) und X-ray diffraction (XRD) the contained phases were determined. The results show a rather good agreement. A strong kinetic inhibition of the beta-phase against dissolution was discovered, which leads to the theory of a stable and a metastable system. The phase diagram of this thesis can be attributed to the stable system.