Hydrogenated amorphous germanium-carbon alloy films have been prepared by magnetron sputtering of germanium in methane-argon gas mixtures. The dependence of structural, optical, electrical and optoelectronic properties on deposition conditions has been investigated, and compared with those of hydrogenated amorphous germanium films deposited by the same apparatus using a hydrogen-argon gas mixture. With increasing r.f. power or sputtering gas pressure, the optical band gap, the activation energy of dark conductivity, the intensity of Ge-C bonds and the concentration of hydrogen bonded to germanium all decrease, while the dark conductivity increases. The photoconductivity shows a maximum of 3 x 10(-7) S cm(-1) at a pressure of 50 mTorr. The photosensitivity is found to be improved by about two orders of magnitude compared with a previously reported value.