The surface reactivity of photogenerated carriers is studied in single crystal- and polycrystalline-ZnO electrodes by means of the photoinduced microwave conductivity technique (PMC). This technique is able to probe the concentration of free carriers photogenerated at a semiconductor-electrolyte (SC/EL) interface. On ZnO single crystals (c-ZnO), PMC experiments show significant accumulation of holes at potentials positive to the flat-band (V-fb) due to slow charge transfer reaction at the SC/EL interface. Differences in PMC response are found with exposed (000 (1) over bar) and (000 (1) over bar) faces. On ZnO polycrystalline electrodes (pol-ZnO), PMC experiments show electron accumulation at negative potentials on the electrodes with higher conductivity. On the other hand, the hole accumulation Signal was less intense than in c-ZnO,indicating a fast hole transfer reaction. Application of a sufficiently high illumination intensity saturates surface reaction sites at pol-ZnO/H2O interface and a hole PMC signal arises accompanied by a diminution in quantum efficiency (eta). Combination of microwave conductivity measurements with electrochemical techniques provides relevant conclusions on bulk and interfacial properties of photoactive materials.