The dependence of electronic conductivity and composition of ceria on oxygen pressure is described by equilibrium equations for a material containing oxygen vacancies, divacancies, cerium interstitials and polarons, where these defects are considered to include distorted zones of material. The analyses takes the extension of simple “point defects” into proper account, whereas earlier analyses have not. It is concluded that available data can be explained by polarons in equilibrium with isolated oxygen vacancies, but not with divacancies or interstitial cerium ions.