The results of diffusion experiments carried out in a tellurium atmosphere suggest that nickel reacts with tellurium vapor forming a second phase at the surface. The effects on the distribution of nickel in PbTe on exposure to a tellurium atmosphere confirm this result as do diffusion studies in the absence of a tellurium atmosphere. These results also suggest an explanation for differences observed in the diffusion of nickel into PbS at low' temperatures (8) in an inert atmosphere and those obtained in the 700~ region under sulfur atmosphere (9). It is also suggested that the diffusing nickel species is different in a tellurium atmosphere than under inert or reducing conditions.The implications of these results on general doping experiments and practice in the lead salts are also of interest. If other impurities in the lead salts behave to change in the crystal stoichiometry as does nickel, methods of doping the lead salts must be reinvestigated. Changes in the crystal composition brought about by changes in the atmosphere above the crystal could induce changes in an impurity distribution which will not return to equilibrium in the time normally assumed reasonable to properly equilibrate crystals. The result will be a nonuniform doping concentration which will obviously affect other measurements.The analysis of the chlorine diffusion data suggest that migration on the anion sublattice occurs via a Frenkel defect, anion vacancies being responsible for motion in lead rich PbTe and anion interstitials in tellurium rich material.