Oxides of silicon and zirconium have been deposited onto silicon, carbon and aluminium substrates by reactive sputtering using a 1 keV argon ion beam and a controlled partial pressure of oxygen. Using RBS, film composition was determined for a given partial pressure of oxygen and different Si or Zr deposition rates. There is evidence of retained argon in the film which is primarily due to argon ions reflected from the sputtered target. Cross-sectional TEM was used to examine the film microstructure and morphology. Both silica films and sub-stoichiometric zirconia films were found to be amorphous,whereas stoichiometric zirconia films were found to be polycrystalline with grain sizes in the range 10-20nm. A model has been developed to predict the composition of deposited films.