Transparent ZnO thin films with a resistivity of about 1−3 × 10 −3 ω cm have been prepared on glass substrates held in a horizontal position by an rf sputtering technique. The deposition experiments were made using a ZnO target containing 2 wt.% In 2O 3. For the films deposited on glass substrates held in a vertical position, a resistivity of 3 × 10 −4 ω cm has been obtained. Air heat treatment experiments have been performed on the indium-doped ZnO films at temperatures between 200 and 350 °C. It was found that electrical resistance of the films deposited on horizontal substrates was stable for the treatment at temperatures below 250 °C. For treatment experiments at 350 °C, an increase in resistance by about 2 orders of magnitude has been observed after a period of 2 h. Experiments further showed that the low resistivity films deposited on vertical substrates were relatively unstable in air even at 200 °C.