Abstract The cubic phase of pure ZrO 2 is thermodynamically stable only at temperatures above 2640 K. This phase, however, can be stabilised at room temperature by doping with oxides of a variety of lower valent metals, e.g. CaO, MgO, Y 2O 3. The doping leads to anion deficient materials with important ceramic and ionic conduction properties strongly depending on the compositional modulation and subsequent structural relaxation. Our latest attempt to determine the defect structure of calcium-stabilised zirconia (Zr 0.85Ca 0.15O 1.85) is the use of the reverse-Monte-Carlo (RMC) simulation technique to fit a model structure to the observed diffuse neutron and X-ray scattering data. In this first attempt to apply the RMC method to a system containing both occupational and displacement disorder, we were able to obtain information about relaxations as well as the oxygen-vacancy and the zirconium-calcium ordering.