Small additions of Si and C have been proven to efficiently improve the creep properties of intermetallic γ-TiAl based alloys. In order to exploit the full potential of these alloying elements, detailed studies of their influence on the phase transformations and the resulting microstructural evolution during processing and heat treatments are a necessity. This work presents a fundamental investigation of the alloying effect of Si in the composition range up to 0.65 at.% on a β-solidifying TiAl alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%). After casting and a subsequent heat treatment at 1200 °C, Si increases the amount of γ phase at the expense of the α2-Ti3Al phase. Due to solid solution strengthening and the precipitation of ζ-Ti5Si3 silicides in the highly Si-alloyed materials, an increase in hardness is observed. As the silicides act as effective obstacles for grain boundaries, these precipitates also control the grain growth kinetics of the α phase and are able to maintain a fine-grained microstructure at 1300°C for holding times up to 20 h. By utilizing in-situ high-energy X-ray diffraction experiments and differential scanning calorimetry, Si is found to exhibit similar effects as Al on the alloying system, effectively increasing solid-solid phase transition temperatures, while simultaneously decreasing the solidus temperature of the materials.