Amorphous ZnO-SnO2 (ZTO) films were radio frequency-magnetron sputtered on glass substrates at room temperature in pure Ar. Postdeposition oxidation and subsequent forming gas reduction annealing (O(2)FG) at 450 degrees C increased electron concentration, mobility, and transmittance of the amorphous ZTO films simultaneously. Neither forming-gas nor oxidizing annealing alone produced the same results. Based on X-ray photoelectron spectroscopy, the second stage forming-gas annealing in O(2)FG possessed the function of reducing Zn2+ to Zn and oxidizing Sn2+ to Sn4+ simultaneously. The reduction of Zn2+ to Zn enhanced the electrical conductivity of ZTO films, while the oxidization of Sn2+ to Sn4+ resulted in improved transmittances. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3528169] All rights reserved.