The electrical activation of Al in ZnO thin films grown by pulsed reactive magnetron sputtering is quantified experimentally for awide range of Al concentrations. Wefind that the activation does not exceed 35% remaining constant for growth temperatures below a certain optimumvalue atwhich the highest free electron density and mobility are achieved. Above this temperature, the Al activation decreases rapidly,while Al is accumulating in the films and their micro-structure as well as electrical properties deteriorate significantly. The analysis of possible mechanisms of Al deactivation suggests that the observed effects may be explained only by considering Al doped ZnO as metastable solid solution showing a tendency to segregation of Al into secondary phases.