In-situ high-energy X-ray diffraction (HEXRD) is a powerful and versatile technique to study the structure of materials in a dynamic context. While radiation sources and hardware components are continuously improved, and data evaluation software programmes become more user-friendly, no consistent solution exists so far to extract information such as phase fraction evolutions from vast series of diffraction patterns recorded during in-situ experiments. Hence, in the first part of the present work, an according data reduction routine is proposed. The second part of this diploma thesis is dedicated to the investigation of a TNM alloy with an increased content of β-stabilising alloying elements Nb and Mo by means of in-situ HEXRD. In a dilatometer setup placed in the synchrotron beam, forged and homogenised specimens were annealed in the (α+β+γ)-phase field region and subsequently subjected to cooling rates ranging from 35 to 1200 K/min. Phase fraction evolutions as functions of temperature and time were correlated with the resulting microstructures. The focus was laid on the γ-TiAl phase, for which a continuous cooling transformation diagram was derived. Additional in-situ heating experiments close to thermodynamic equilibrium conditions, combined with quantitative metallography on heat-treated and water-quenched specimens, provided ancillary information for temperature calibration. Intermetallic TNM alloys exhibit excellent processing characteristics due to a high amount of disordered β-phase at hot working temperatures. In a post-forging multi-step heat treatment, balanced mechanical properties are adjusted. The performed in-situ diffraction experiments offered a deeper insight into the phase transformation behaviour of the investigated type of multi-phase alloy. The gained information, which is essential for the optimisation of heat treatment steps, is not accessible with conventional characterisation techniques.