Zinc oxide (ZnO) thin films were prepared by d.c. (direct current) or r.f. (radio frequency) magnetron sputtering on glass substrates. The structural, electrical and optical properties of the films were studied. It has been found that most films produced by d.c. sputtering are not electrically conductive or have a high resistance above 10 Omegam, while the films produced using r.f. sputtering are significantly more conductive. The optical transmittance at 550 nm is around 80% or higher for most films. The energy band gaps of the ZnO films prepared by d.c. sputtering are smaller than the films prepared by r.f. sputtering. It is also found that the films prepared using an electrical bias have higher resistivity than those produced without bias. It has been observed by SEM that the conductive films show less porosity between the grains than the poor conductive films. Conductive ZnO films show a smaller d spacing than the non-conductive films. The results reveal that crystal microstructure and density of the ZnO films affect their conductivity. (C) 2003 Elsevier B.V. All rights reserved.