We have studied the effect of codoping ZnO with Al and In using composition spreads prepared by off-axis rf sputtering. We find a large improvement in the electrical conductivity of doped ZnO and attribute this to size compensation, as inferred by measuring the volume of the unit cell as a function of composition. Adding a small amount of In to Al-doped ZnO results in an increase in the fraction of Al dopant atoms that are activated, and the mobility remains high as well. Codoping allows for a higher conductivity than single-doping in films deposited at a modest temperature with no post annealing. (C) 2008 Elsevier B.V. All rights reserved.