Lattice parameter data of cubic phases and cube roots of unit cell volumes of tetragonal phases in homogeneous ZrO2‐containing solid solutions were compiled to examine the validity of Vegard's law. Except for ZrO2–CeO2and ZrO2–UO2systems, the data for cubic phases were expressed by the equationd=asX+b, whered,as,X, andbdenote the lattice parameter, a constant depending on dopant species, the dopant content, and a constant independent of dopant species, respectively. For tetragonal phases, the cube roots of unit‐cell volumes could be fitted by a similar equation except for the data in the ZrO2–MO2systems (M = Ge and U). The constantaswas calculated using an ion‐packing model and was independent of the defect cluster models. The calculatedasis close to the experimentally observed one, although the former is slightly smaller than the latter in the ZrO2–MOusystems (u= 1 and 1.5). This difference was ascribed to the lack of consideration of the ionic distortions from the ideal sites of the fluorite