Zinc tellurite glasses appear to be excellent candidates for hosting rare earth ions since they provide a low phonon energy environment to minimize non-radiative losses as well as possessing good chemical durability and optical properties. The optical behavior of the rare earth ion can be manipulated by modifying its local environment in the glass host. We report measurements of the emission lifetime, optical absorption, and vibrational density of states of the glass system (ZnO)x(ZnF2)y(TeO2)1-x-y doped (0.1 mol%) with a series of rare earths. Phonon sideband spectroscopy has been successfully employed to probe vibrational structure in the immediate vicinity of the rare earth ion. We observe a significant increase in the emission lifetime (from ∼ 150 μs to 250 μs) of Nd3+ with increasing fluorine substitution. © 1997 Elsevier Science B.V.