We describe the development of a three-dimensional (3D) model which provides a simple explanation of virtually all the features that occur in the complex diffuse X-ray diffraction patterns of an yttriastabilized cubic zirconia. The model consists of two stages: a scheme for ordering the oxygen vacancies, followed by the relaxation of the cations around these vacancies. Monte Carlo simulation is used in both these stages, followed by direct computation of the diffraction patterns from the resulting lattice realizations. The model which at present best fits the observed scattering patterns is one in which the oxygen vacancies order in such a way as to avoid nearest-neighbor (\frac{1}{2}\langle 100\rangle) pairs, next-nearest (\frac{1}{2}\left(\begin{array}{lll}1 & 1 & 0\end{array}\right)) pairs, and third-nearest (\frac{2}{2}\left(\begin{array}{lll}1 & 1 & 1\end{array}\right.) pairs across empty cubes of oxygens, but allows third-nearest (\frac{1}{2}\left(\begin{array}{llll}1 & 1 & 1\end{array}\right)) pairs across cubes of oxygens containing the cations. These vacancy pairs, which essentially provide octahedral coordination of the enclosed cation, are therefore present almost entirely as either single isolated octahedra or neighboring (\langle 110\rangle) pairs of octahedra. A comparison of the diffuse patterns with those from a calcia-stabilized zirconia is made. 1993 Academic Press, Inc.