A deep level transient spectroscopy (DLTS) study of 450°C annealed Czochralski silicon is presented. Particular attention is given to the relative concentrations of the two thermal donor energy levels Ec-0.15 eV and Ec-0.07 eV. Relative concentrations of the Ec-0.15 eV and Ec-0.07 eV energy levels indicate that there are fewer of the more shallow level. Also there is anomalous motion of the energy levels with anneal time indicating the gradual accretion of the thermal donor complexes. The suggested correlation with infrared absorption (IR) studies is that the nine double-donor defects found by IR form sequentially in the material and the DLTS energy level obtained merely reflects the most abundant of the nine distinct complexes. This indicates that the nine thermal donors are formed by the addition of some constituent to an earlier complex. As the thermal donor complex accretes the associated energy levels change, moving to shallower energies as anneal time increases. These findings tend to contradict the simple thermal donor models.