We investigate the defect properties of group-I elements such as Li and Na in ZnO through fi rst-pri nci pies calculations. Compared with group-V elements such as N, P, and As, Li and Na dopants at substitutional sites have shallower acceptor levels, but, these acceptors are mostly compensated by coexisting interstitial donors. Our calculations show that a codoping technique with hydrogen severely suppresses the concentration of interstitial donors, and greatly enhances the solubility of group-I dopants via the formation of hydrogen-acceptor complexes. The hydrogen-passivated acceptors easily recover the electrical activity by post-annealing, and thus lowresistivity p-type ZnO is achievable with dopants different from group-V elements. (c) 2005 Elsevier B.V. All rights reserved.