Films of Ta2O5 have been prepared by reactive sputtering of tantalum in argon-oxygen dc glow discharges. Such films are of interest for capacitor dielectrics and optical waveguides, and for both applications the films must be free of pores. The effects of sputtering pressure on the density, refractive index, and dielectric constant of the films has been investigated. In the case of metal films, significant decreases in density are caused by increasing the sputtering pressure, a similar effect has been observed in the Ta2O5 case. The density decreases from 7.8 to 5.9 g/cm3 as the pressure is increased from 25 to 60 mTorr, and there are associated changes in related properties. The sputtering pressure can be controlled by adjusting the argon-oxygen mixture, but this also changed the deposition rate. Optical emission or mass spectrometry is used to ensure that the oxygen concentration is high enough for the deposition of Ta2O5 films rather than oxygen-doped tantalum; no emission from oxygen is observed until this concentration is reached.