The study of the influence of oxygen partial pressure on the properties of indium. tin oxide (ITO) thin films deposited onto polymer substrates at room temperature is presented. Two types of polymer substrates were used: polyethylene terephthalate (PET) and polyethylene naphathalate (PEN). The deposition technique used is the radio frequency plasma enhanced reactive thermal evaporation of a 90% In-10% Sn alloy in the presence of oxygen. The oxygen partial pressure varied between 2 X 10(-4) and 4 X 10(-3) mbar and the average thickness of the films is approximate to40 nm. Results show that ITO thin films deposited on polyester (PET) at the optimum oxygen partial pressure of 6 X 10(-4) mbar exhibit the following characteristics: an average total visible transmittance of 80% and an electrical resistivity of 1.9 X 10(-2) Ohm cm.