Electrical and optical characterisation of hydrogenated amorphous silicon oxygen alloy thin films (a-SiOx:H, x < 2) grown in a single chamber radio frequency plasma enhanced chemical vapour deposition (PECVD) system at a high substrate temperature of 300 degrees C is presented. The samples were investigated by Fourier transform infrared spectroscopy (FTIR), optical transmission, the constant photocurrent method (CPM), conductivity and steady-state photoconductivity measurements. With increasing oxygen concentration, the Tauc gap increases from 1.69 to 2.73 eV. The sample with an oxygen concentration of 26.2 at% and a reasonably high bandgap of 2.18 eV shows photoconductivity comparable to that of pure a-Si:H films. The Urbach parameter (E-0) increases almost linearly with oxygen concentration whereas the dangling bond defect density is found to be saturating at a value of about 7.1 x 10(16) cm(-3). One of the highly alloyed samples with E-0 = 123 meV exhibited a detectable photosensitivity. (C) 2005 Elsevier B.V. All rights reserved.