Absorption transition from the ground H-6(15/2) state to the superimposed (H-6(9/2), F-6(11/2)) states, inverse transition of which leads the Dy3+: 1.3 mum luminescence, in noncrystalline Ge-As-S alloys is dramatically modified upon a least compositional adjustment by Ga and CsBr. The change in oscillator strength of the Dy3+: H-6(15/2) <----> F-6(11/2) transition in our modified glasses is more sensitive to the local coordination structure of rare-earths compared to other hypersensitive transitions that have been previously observed in solid-state dielectric hosts. Preferential highly symmetric coordination in nearest-neighbouring shell of dysprosium with bromine in [GaS3/2Br](-) complexes, which are formed spontaneously during vitrification process of the host materials, is believed to be responsible for the behaviour. (C) 2005 Elsevier B.V. All rights reserved.