Silicon, heat treated to temperatures above 1000 °C followed by a rapid quench, shows a donor activity due to the so called thermally induced donors (TID’s). In this paper it is shown that for samples treated in this way the TID effect and the behavior of the resistivity as a function of time after quenching is exactly the same as for similar samples which have been deliberately doped with iron. Hall measurements reveal the same donor levels atEv+0.40 eV in both cases indicating that unintentional Fe contamination may have taken place in the former samples during high‐temperature treatment. An extra argument in favor of the opinion that TID’s do not originate from an intrinsic effect but are due to unwanted doping is given by the fact that the entire TID effect can be eliminated by treating the samples in a H2flow containing a few percent HCl and 100 ppm O2. This mixture, which has no etching capacity, has a strong gettering effect for Fe because of the high volatility of the iron chlorides in the gas phase. Samples heated in this gas mixture and quenched rapidly remain completely free of TID’s and show no other donor activity within the accuracy of the measurements.