Hydrogenated amorphous germanium-sulphur (a-G1−xSx: H) alloys have been prepared by the glow-discharge technique from mixtures of hydrogen, germane and sulphur hexafluoride gases. The sulphur content of the films prepared at the powered and the unpowered electrodes varied between 2 and 30 at.%. Addition of sulphur decreases the activation energy of the conductivity from a typical value of 0.5 eV for hydrogenated amorphous germanium (a-Ge: H) to minima of 0.14 and 0.28 eV for the alloys deposited on the unpowered electrode and powered electrode respectively. Concurrent with this, for the cathode-deposited films, the ημτ product of electrons is increased by one order of magnitude to a maximum of 2 × 10−6cm2V−1, and the ambipolar diffusion length decreases from 470 to 350 Å. From these trends, and with a consideration of various additional experimental findings, it is inferred that n-type doping occurs when sulphur is added to a-Ge: H. Sulphur incorporation improves the quality of films prepared at the unpowered electrode as is evident from the optoelectronic properties; the ημτ product increases by more than three orders of magnitude to 10−6cm2V−1, whereas the ambipolar diffusion length is nearly constant at around 400 Å.