Indium-tin-oxide (ITO) is a transparent conducting material which is deposited as a thin film on glass substrates for use in opto-electronic devices. However, there are several applications for which ITO films on polymeric substrates are desirable. The sheet resistance, optical transmittance, and microstructure of as-deposited ITO thin films on unheated polyethylene terephthalate substrates were studied using rf sputter deposition. During separate deposition runs, the partial pressure of oxygen was varied from 5% to 20% and the deposition time was varied from 15 to 120 min. No significant variations are observed in the sheet resistance with respect to oxygen partial pressure; however, changes in sheet resistance were observed in ITO films deposited on different substrates for short deposition times (15 min). Additionally, the thickness of the film (assumed to be proportional to the deposition time) is shown to have a considerable impact on the sheet resistance and the optical transmittance. The x-ray diffraction data coupled with the sheet resistance data suggest that ITO films exhibiting a preferred (400) orientation have the highest sheet conductance. The changes observed in the sheet resistance, optical transmittance, and the refractive index due to the thickness of the film or oxygen partial pressure during deposition are explained on the basis of surface roughness, free carrier concentration, and the microstructure of the film.