The effect of erbium concentration on thermal stability and quantum efficiency of excited states was studied using DSC, XRD and optical methods. A large value of stability parameter AT close to 118 degreesC has been obtained for fluoroindate glass singly doped with 2 mol% of erbium ions. With increasing erbium concentration stability parameter extremely decreased and several crystalline peaks appeared, what was stated by X-ray diffraction. Also, the strong luminescence quenching is observed. This behavior is consistent with nonradiative processes related to the Er-Er interaction. (C) 2003 Elsevier B.V. All rights reserved.