Magnetic susceptibilities of Ce y U 1−y O 2+x (x ⪋ 0) solid solutions with fluorite structure were measured from 4.2 K to room temperature. An antiferromagnetic transition was observed for the oxygen-stoichiometric ( x = 0) and deficient ( x < 0) solid solutions with y values less than 0.40. The Néel temperatures of oxygen-deficient solid solutions are lower than those of oxygen-stoichiometric solid solutions with the same y value, which indicates that the formation of oxygen vacancies results in weakening the magnetic interactions between uranium ions. From the analysis of magnetic susceptibility data, it was found that some cerium ions are reduced to trivalent state for oxygen-deficient solid solutions, while some uranium ions are oxidized to pentavalent state for oxygen-excessive solid solutions.