Abstract Capacitance measurements made on single crystal zinc oxide electrodes in contact with aqueous electrolytes are reported. Over a wide range of bias and bulk donor density, the results are in almost quantitative accord with predictions of the simple Poisson-Boltzmann (Poisson-Fermi in the degenerate case) equation. This is shown to imply the complete absence of surface-state effects in this system. A very sharp discontinuity in the flat-band potential is observed at bulk electron densities in the range from O.6 × 10 18 to 2 × 10 18 donors/cm 3. This and other effects, arising under varying surface treatments, are discussed in some detail.