Hydrogen ion implantation technique was applied to introduce hydrogen into ZnO. ZnO specimens with different electrical conductivity were prepared by the addition of some dopants. The elastic recoil detection analysis was used to measure the content of hydrogen in ZnO before and after the hydrogen ion implantation. The electrical conductivity of ZnO was increased greatly by the hydrogen ion implantation. The increase of the electrical conductivity varied with the dopants. The greatest increase was about 9 orders of magnitude in the highly resistive Cu-doped ZnO. The mechanism for such a hydrogen effect was discussed. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.