Highly transparent ZnO thin films were prepared by calcining ZnO 2 precursor films deposited from aqueous solutions. The transparency of the resultant ZnO thin films, which was directly associated with that of the precursor films, depended on the kind of zinc source, the composition of the starting solution, and the deposition temperature of the ZnO 2 precursor. The most highly transparent ZnO thin film was obtained by calcining a ZnO 2 precursor film which was deposited from a solution containing 0.01 moldm 3 of ZnSO 4 , 1.8 moldm 3 of H 2 O 2 , and 2.1 moldm 3 of NH 3 at 303 K. Doping of a ZnO thin film with indium was attempted by an immersion of the thin film in an In NO 3 3 solution and a subsequent heat-treatment at 773 K in a N 2 atmosphere, which resulted in an electrical conductivity of 535 Sm.