The effect of low amounts of dysprosium, samarium and manganese (0.1 - 0.5 at.%) on structure and optical properties of chalcogenide glassy semiconductors As₂S₃ and As₂Se₃ is investigated. The fundamental absorption edge shifts to longer wavelength with doping; the largest shift was observed for As₂S₃ glasses doped with Dy and Mn. A correlation of the properties with medium range order and electronegativity of impurity atoms was revealed. Photodarkening relaxation under light exposure of a-As₂Se₃ amorphous films doped with 0.5 at.% of metals Sn, Mn, Sm and Dy was studied in dependence on the impurity and thermal treating. Both factors reduce photodarkening and the degree of reduction depends on the sort of impurity. The relaxation process may be described by a stretched exponential with the dispersive parameter 0.5 <α[removed]