Intermetallic γ-TiAl based alloys represent novel high-temperature materials, which allow to meet today's increasing demands for engine light-weighting in both aero and automotive industries. Characteristic material properties are their low density, high strength and stiffness retention at elevated temperatures as well as high creep and oxidation resistance. These properties render this material applicable as turbine blades in the low pressure turbine of latest generation aero engines as well as turbocharger turbine wheels of automotive engines. In order to further increase the maximum service temperature of the established TNM alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (at.%), in the present thesis fundamental investigations of the effects of microalloying with C and Si as well as effects of Mo have been conducted. This approach required the detailed microstructural analysis reaching the nanometer scale by the use of atom probe tomography, transmission electron microscopy, transmission electron backscatter diffraction and X-ray diffraction complemented by ab initio calculations. Moreover, the foundations of the prevailing deformation phenomena were assessed by nanoindentation, tensile creep testing and mechanical spectroscopy. The investigations performed revealed that a significant increase in mechanical properties of advanced TiAl alloy can be achieved by small additions of C, which hardens the microstructure via solid-solution hardening. In this case the C concentrations dissolved in the various phases were determined by atom probe tomography and then correlated to the local hardness revealed by nanoindentation. Small amounts of Mo were shown to kinetically retard decomposition reactions of the βo phase due to its pronounced partitioning behavior between the phases present. Additions of Si yield the precipitation of silicides that can stabilize lamellar microstructures against degradation. Thereby, novel precipitation phenomena, which can only occur in β-solidifying TiAl alloys, were clarified. Moreover, the predominant deformation mechanism of nano-lamellar microstructures at elevated temperatures were analyzed. The comparative use of tensile creep experiments, mechanical spectroscopy and transmission electron microscopy revealed that dislocation climb along lamellar interfaces is governed by a jog-pair formation process. The studies conducted and presented within this thesis contribute to the detailed understanding of alloying effects in latest generation γ-TiAl based alloys. This fundamental insights into the effects of alloying elements can be applied for the definition of enhanced alloy compositions and microstructures.