Hydrogenated amorphous silicon oxygen alloy (a-SiO:H) films have been prepared by RF plasma enhanced chemical vapour deposition from (SiH4 + CO2 + H-2) gas mixture. Films have been characterized, in detail, by electrical, optical as well as structural studies. The effect of the oxygen incorporation into the Si-network was studied by controlling various deposition parameters e.g., CO2 to SiH4 flow ratio, H-2 dilution of the plasma, total flow rate of the reacting gases, RF power applied to the electrodes, working gas pressure in the plasma chamber and the substrate temperature. Optical gap of the films increased due to the incorporation of O, and a lowering in photoconductivity with optical gap widening was monitored. Increasing polyhydrogenation at higher O-content resulted in a rise in defect density. O-incorporation into the Si-network increased the light-induced degradation in photoconductivity. (C) 2000 Elsevier Science B.V. All rights reserved.