Abstract The characteristics of reactively r.f.-sputtered tantalum oxide films formed under a variety of sputtering conditions have been studied. Sputtering gas composition, gas pressure, r.f. power and substrate temperature have been varied to study their effects on the refractive index, deposition rate, film structure and atomic composition of the deposited tantalum oxide films. For a certain total presssure and gas composition of the reactive atmosphere, the deposition rate and refractive index will reach a peak value. An increase in the r.f. sputtering power will increse the deposition rate, but the refractive index of the deposited films will be decreased. An increase in the substrate temperature will lead to films of higher refractive indices. Correlations between the atomic composition and the refractive index have been studied and it is found that, the larger the atomic ratio of oxygen to tantalum, the higher is the refractive index.