A study was made to obtain an understanding of the factors involved in the “poisoning” phenomena which are responsible for high reverse currents in silicon junctions. A tentative model has been proposed which describes the conditions by which fast diffusing metal impurities can precipitate within junction regions under certain high‐temperature ambients and yet not under others. Oxygen appears to be a significant factor which can control the degree and location of such metal precipitation. The role of oxygen was investigated in its relation to impurities and defects in silicon. Its importance was demonstrated by copper‐oxygen reactions, oxygen tie up complexing to form donor states, and by the effect of induced dislocations.