Intermetallic alloys based on the γ-TiAl phase are used as structural materials for turbine blades in the latest generation of aircraft engines. To further propagate and enhance their applicability and efficiency, an increased operation temperature is required. This can be realised by proper alloying development, through optimisation of processing conditions as well as by adjustment of microstructures which provide balanced mechanical properties. Within this thesis, the effects of additional alloying elements on the established TNM alloy system are investigated. The elements Al, C and Si are known to stabilize the α-phase, to increase the hot-temperature strength and to enhance the creep properties through the existence of p Ti2AlC and ζ-Ti5Si3 precipitations. On the other hand, the elements Zr and W are considered as solid-solution strengtheners of the γ-phase and they furthermore influence the fraction of the βo phase. The presence of a sufficiently extended single α-phase field region enables a simple two-step heat-treatment to adjust a beneficial fully-lamellar microstructure. Within this microstructure, consisting of colonies of alternating α2- and γ-lamellae, the movement of dislocations is hindered by the presence of numerous α2/γ as well as γ/γ interfaces, which control the creep properties. To evaluate the effect of the lamellar spacing, precipitations and their interaction with dislocations, creep and tensile tests combined with complementary transmission electron microscopy investigations were conducted. Based on this fundamental investigation, the application temperature of modified TiAl alloys could be enhanced and the thermal stability of the microstructure improved.