The space charge density at the surface of yttria-stabilised zirconia (YSZ) containing 2–10 mol% yttria was determined from differential capacity measurements of YSZ/gold interfaces at 750–850 K. The oxygen vacancy concentration in the sub-surface layer was determined as a function of temperature and bias potential by fitting the experimental data at positive bias potentials to predictions based on a Boltzmann-type expression. The vacancy concentration at the interface increases with decreasing bias potential, and reaches a temperature-dependent maximum value at negative bias potentials. The thermal activation parameter for this process is 0.27–0.46 eV. A thermodynamic model for the near-surface defect structure of YSZ is proposed, in which oxygen vacancies are assumed to be distributed randomly over a fraction of the total number of oxygen sub-lattice sites. The fraction of sites that is available for oxygen vacancy distribution increases with yttria content and temperature.