We study the effect of Sn impurities on the generation and annealing kinetics of oxygen-containing thermal donors formed during a 450 degree C anneal step. Special attention is given to the influence of a thermal pre-heat treatment at 300 degree C and the important role played by oxygen micro fluctuations. The latter act as precursors for the formation of thermal donors. The qualitative theoretical model, developed for explaining the experimental results in Sn-free Czochralski Si and based on hetero- and homogeneous precipitation processes, also allows one point out a beneficial role of Sn doping on the thermal donor properties.