Hydrogenated amorphous silicon-oxygen alloy (a-SiOx: H) films were deposited by r.f. glow-discharge decomposition of a SiH4 + CO2 gas mixture at a substrate temperature of 300degreesC. The optical band gaps of the samples were found to change between 1.65 and 2.73 eV by varying the oxygen content from 4.5 to 64.2 at%. While both the room-temperature photo- and dark-conductivity decrease with oxygen alloying, for oxygen contents below 11.9 at% the measured conductivities are comparable to those of unalloyed a-Si:H. In contrast, the deep-defect density and Urbach parameter continuously increase with oxygen alloying. (C) 2003 Kluwer Academic Publishers.