The monograph presents the results of experimental observation using the method of emission Mössbauer spectroscopy of impurity atoms that are part of anti-structural defects in crystalline and glassy chalcogenide semiconductors (crystalline chalcogenides of lead and solid solutions based on them, glassy chalcogenides of arsenic and germanium). The use of the emission version of Mössbauer spectroscopy made it possible to stabilize the impurity atoms in unusual positions after a chain of radioactive decays of mother nuclei. The book is intended for researchers, doctoral students and graduate students specializing in the physics of impurity semiconductors.