The defect structure of La 2− x Ca x CuO 4− y superconductors was examined by fitting electrical property and oxygen nonstoichiometry data to a variety of models. Those involving charged oxygen vacancies, whether isolated or associated, were inadequate for simultaneously describing the dependence of both properties on doping level and oxygen partial pressure. Neutral oxygen vacancies and their association with dopants were invoked, resulting in a successful reconciliation of all existing experimental data. The defect types and configurations can be rationalized in terms of the bond-mismatch between the (LaO) 2 and CuO 2 layers in the La 2CuO 4 structure. The defect associates proposed may be responsible for the suppression in T c relative to the La 2− x Ba x CuO 4− y and La 2− x Sr x CuO 4− y systems.