Under an applied AC voltage and upon UV illumination, ZnO nanowires exhibit a continuous transition from positive to negative photoconductivity as a function of the AC frequency. ZnO nanowires also display a frequency-driven metal-insulator transition at room temperature. The responsible mechanism for both transitions has been attributed to a competition between bulk conduction and surface conduction. The frequency-driven bulk-to-surface transition of conductivity is expected to be a generic character of semiconductor nanostructures with the large surface-to-volume ratio.