The aim of this work is to study different compositions in the Ga-Ge-Sb-S system for the definition of two compositions compatible with the elaboration of a single-mode fiber at the 1.55 mu m telecom wavelength. The variations of the glass transition temperature (T-g), the dilatation coefficient (alpha) and the refractive index (n) have been studied for two glasses series: GaxGe25-xSb10S65 (series 1), Ga5Ge25-xSb10S60+x (series 2). This study has lead to the choice of the Ga4Ge21Sb10S65 composition as clad glass for the preparation of the single-mode fiber and Ga5Ge20Sb10S65 composition as the core. The discrepancies for the studied parameters between the core and clad compositions are the following: Delta alpha < 2%, Delta Tg < 3% and Delta n = 5 x 10(-3). Finally a single-mode fiber has been prepared by the rod in tube technique. (C) 2007 Elsevier Ltd. All rights reserved.