In this work the anomalous formation kinetics of old thermal donors (TD) observed in the initial stage of formation in pre-heat-treated carbon-rich samples are studied. Computer simulations applying a reaction scheme where the TD-complexes are subsequently formed by single oxygen diffusion reveal that the experimental results can only be acounted for if the pre-existing TD-core involves three oxygen atoms and has a concentration in the range 1013-1014cm−3. Carbon is found to have a major influence on the production rate of pre-existing cores.