Abstract Zinc and lead chloro-tellurite glasses are candidates for hosting rare earth ions, due to a rare earth ion solubility as high as 10 mol% and maximum vibrational energies lower than 800 cm −1, combined with a chemical durability superior to that of halide glasses and optical transparency from the visible to the middle infrared. A series of glasses in the TeO 2–ZnO–ZnCl 2 system, doped with 1–10 mol% ErCl 3, were prepared and heat treated in air between 350 and 500 °C. The early stages of the formation of the crystalline phases formed within the TeO 2-based glass matrix were determined in situ by time-resolved X-ray powder diffraction, recorded during thermal treatment, using a synchrotron radiation source. Only two crystalline phases, TeO 2 and Zn 2Te 3O 8, were identified. Extended X-ray absorption fine structure spectroscopy above the Er L III edge was used, to characterize the Er 3+ environments in these oxyhalide glasses, before and after heat treatment. The presence of erbium was found to affect the crystallisation process, since other crystalline phases were formed, which could not be identified, but which appear to include Er 3+ ions.