Electrochemical quartz crystal microgravimetry studies of porous nanocrystalline ZrO 2 electrodes in acetonitrile containing 1M LiClO 4 show that surface electronic states can be accessed at potentials as far positive as 0V versus Ag/AgCl, as evidenced by uptake of charge-compensating cations. A much higher density of surface states is encountered beginning at about -1.3V. Based on previous work with TiO 2 , SnO 2 , and ZnO, this potential is tentatively identified with E c b for ZrO 2 and is about 0.5V more negative than E c b for TiO 2 . In water, cation uptake is replaced by efficient reduction of H 3 O + or water to hydrogen, a finding that has interesting parallels in radiation chemistry. Identifying the onset potential for hydrogen evolution with either E c b or a potential characteristic of a high density of trap states, the value obtained is about 0.3V negative of E c b for TiO 2 . Like the conduction band edge energy for titanium dioxide, the putative E c b value for ZrO 2 shifts negatively with increasing pH. Comparisons of surface-based ligand-to-metal charge-transfer band energies point to an E c b value for colloidal ZrO 2 in water that is about 0.4V negative of the value for colloidal TiO 2 . Consistent with three recent literature reports, empty states should lie low enough in energy to permit efficient injection from photoexcited dyes under certain conditions.