The study of point defect complexes and thermal double donors (TDDs) in Silicon, has been of great interest in the field of semiconductor physics for several decades. However, due to a growing environmental awareness and increasing demand for high-efficiency and low-cost solar cells, the research activity in this field is once again flourishing. In this work, n-type Cz-Silicon with high and low carbon concentrations have been sequentially annealed in the temperature range of 450-550°C, and investigated using FTIR spectroscopy. MeV electron irradiation has facilitated a detailed study of vacancy-oxygen (VOn) and carbon-related complexes. Their formation and evolution during thermal treatments have been studied, and their relation to TDD formation has been addressed. TDDs were found to form at 450 and 500°C, while being unstable at 550°C. FPP resistivity measurements provided an estimate for TDD concentrations after thermal treatments. The presence of high carbon concentrations were found to strongly inhibit TDD generation. Cz-Si with carbon concentrations of 2.51017cm..3 were found to reduce the final TDD concentration by factors of 7 and 30 when annealing at 450 and 500°C, respectively, relative to carbon-lean samples. The obtained results suggest that carbon specifically impedes the formation of TDD3. Formation kinetics confirmed a sequential formation of the larger CsO3i complexes by prolonged annealing of the carbon-rich samples at the highest temperatures, while the smaller CsOi centres became unstable. The direct involvement of VOn centres in TDD formation was ruled out. The presence of both VOn and CsOn defects were found to consume oxygen and act as traps for migrating oxygen atoms during annealing, effectively reducing TDD formation. PL measurements have been correlated with IR measurements in terms of carbon-complexes and formation of TDDs. Zero-phonon and phonon replica luminescence lines of TDDs and irradiation induced carbon-complexes have been identified.