Abstract Undoped and Pr 3+-doped Ge x S y I z glasses, with atomic concentrations of germanium 0.33 ≤ x ≤ 0.43 and iodine 0 ≤ z ≤ 20, respectively, were synthesized and their optical and thermal properties measured. The properties followed the same compositional trends as the binary Ge x S 1 - x glasses, for which 0.33 ≤ x ≤ 0.43. The compositional dependences of the absorption edge and rare-earth solubility in Ge x S y I z glasses can be presented with a single parameter p = (y + z 2 ) x . Addition of iodine to the Ge-rich Ge x S 1 - x -base glass system resulted in increasing the optical bandgap energy and decreasing the glass transition temperature. Thermal stability of the Ge x S y I z glasses is a function of both S Ge ratio and iodine concentration. XPS and Raman spectroscopic measurements were conducted to obtain information about the structural changes accompanying the compositional variations. Properties-structure correlations are discussed. The results of the present study were used for selection of a compositional range within the Ge x S y I z glass system with optimal properties required for Pr 3+-doped fiber amplifier application.