The defect structure of 7 mol% calcium-stabilized zirconia (CSZ) is described in terms of a correlated distribution of microdomains within the cubic matrix of CSZ. It is shown that the defect structure is very similar to that of 15 mol% CSZ. The defect structure consists of two types of defects: microdomains based on a single oxygen vacancy with relaxed neighbouring ions and microdomains based on a pair of oxygen vacancies separated by a3~/2/2 along (111). Calculations show that a tetragonal distortion cannot explain the observed diffuse scattering. Several arguments suggest that the defect structure is not that of the qO t phase: first, the similarity of diffuse scattering of yttrium-stabilized zirconia, for which no ~ phase exists; second, the diffuse scattering of CSZ is almost identical from 4 mol% CSZ up to 20 mol% CSZ; third, the diffuse scattering is temperature dependent; and fourth, a direct comparison of single-crystal intensities of the qo~ phase with the intensity of diffuse scattering.