Homogeneous germanium oxy-sulfide (1 − x)GeS2–xGeO2 glasses were prepared from 0 ⩽ x ⩽ 1 by rapidly quenching melts to room temperature showing no evidence of phase separation all across the compositional range. A non-linear variation of the glass transition temperature (Tg) was observed where Tg increased initially with small amounts of GeO2 up to x = 0.1, remained approximately constant over the range of 0.2 ⩽ x ⩽ 0.8, and then increased again at the end of glass forming range (x ⩾ 0.8). Bridging Ge–S–Ge and Ge–O–Ge vibrational stretching modes were observed in both the Raman and Infrared spectra. Bending modes of the S–Ge–O linkages were also observed in the Raman spectra. From these results, it is suggested that the addition of GeO2 to GeS2 glass causes the transformation of GeS4/2 units to mixed oxy-sulfide GeS3/2O1/2, GeS2/2O2/2, and GeS1/2O3/2 structural units at progressively higher GeO2 contents. While the structure of these glasses apparently changes continuously and smoothly with composition, the non-linear changes in the physical properties like the Tg are not fully understood, although other common examples of such non-linear behavior are well known in simple binary alkali silicate glasses.