The defect which appears inp‐type silicon quenched from 870° to 1070°K is characterized by an ionization energy about 0.4 eV from the valence band, by an activation energy for motion about 0.3 eV, and by a diffusion constant of 1.2×10−7cm2/sec at room temperature. The principal feature of the defect during room temperature annealing is that it fits into dimension‐dependent kinetics. This indicates that the process of elimination of the defect occurs at the surfaces of the samples and that in thicker samples only a fraction of the defects succeed in reaching the surfaces. The high diffusion constant makes it plausible that the defect is an interstitial silicon. Mayburg in 1956 concluded from a series of experiments that interstitials exist in large densities (∼1016cm−3) in silicon crystals as grown; he also considered their motion to the surfaces. We believe that the present experiments point to activation of interstitials during the heating process. They may have existed in an electrically inactive form in the crystals as grown, i.e., near some imperfection. Their random diffusion brings them to the surfaces where again they lose their donor properties.