We have used conducting atomic force microscopy ͑CAFM͒ to study the morphology and electronic behavior of as-received and air-annealed ͑0001͒ Zn-and ͑0001͒ O-polar surfaces of bulk ZnO. Both polar surfaces exhibit relatively flat morphologies prior to annealing, which rearrange to form well-defined steps upon annealing in air at 1050°C for 1 h. Long-term exposure to air results in surface layer pitting and the destruction of steps for both the as-received and air-annealed ͑0001͒ surfaces, indicating its enhanced reactivity relative to the ͑0001͒ surface. CAFM I-V spectra for polar surfaces are similar and indicate Ohmic to rectifying behavior that depends on the maximum applied ramp voltage, where higher voltages result in more conducting behavior. These data and force-displacement curves suggest the presence of a physisorbed H 2 O layer, which is removed at higher voltages and results in higher conduction.