The paper shows results of investigation of thermal and optical properties of oxyfluorotellurite (65 − x)TeO2–20ZnF2–12Pb2O5–3Nb2O5–xDy2O3 (x = 0.5, 2 and 5) glass systems. Thermal stability and the onset of crystallization of the materials were monitored by differential thermal analysis (DTA). It was found that characteristic parameters, namely glass transition temperatures (Tg), onset of crystallization temperatures (Tc) and thermal stability criteria ΔT and H’ increased with increasing Dy2O3 content indicating that the incorporation of dysprosium ions improves substantially thermal stability of glass system under study.