Buried channel waveguides have been fabricated by ion implantation of Plasma-enhanced chemical vapour deposition (PECVD)-grown silica-on-Si. Post-implantation annealing was observed to have a significant influence on waveguide loss as measured at a wavelength of 1550 nm-loss decreased abruptly from an as-implanted value of ~1 dB/cm to ~0.15 dB/cm following a 400 o C/ h annealing cycle. However, annealing at greater temperatures (500 o C) yielded a value comparable to the as-implanted result. For the present paper, the various factors that potentially influenced the observed loss behaviour have been addressed. Such factors included thermally-induced changes to density and refractive index, mode profile spreading and subsequent interaction with the waveguide surface, precipitation of the implanted ions and annealing of both intrinsic and implantation-induced defects. The observed loss behaviour has been attributed to a combination of effects dominated by a reduction in implantation-induced defect concentrations (300-400 o C), where such defects acted as scattering and/or absorption centres, and mode profile spreading (400-600 o C) due to a reduction in refractive index.