Abstract Magnetic susceptibilities of Y y U 1− y O 2+ x solid solutions with fluorite structure were measured from 4.2 K to room temperature. For the solid solutions with y values less than 0.50, an antiferromagnetic transition was observed. The Néel temperature decreased with decreasing uranium concentration, but in a different manner from that of (U,Th)O 2 or (U,Zr)O 2 solid solutions. The magnetic moment and Weiss constant were determined in the temperature range in which the Curie-Weiss law holds. With decreasing uranium concentration, the magnetic moment decreased more rapidly compared with those for (U,Th)O 2 and (U,Zr)O 2 solid solutions. The Weiss constant monotonously decreased with decreasing uranium concentration. The oxidation state of uranium in the solid solutions was discussed by means of the magnetic susceptibility data for specimens with various x and y values. The substitution of Y 3+ for U 4+ in UO 2 proved to show the magnetic dilution effect with simultaneous oxidation of U 4+ to U 5+. The critical concentration for antiferromagnetism, C 0, for oxygen-stoichiometric solid solutions, (U,Y)O 2.0 was estimated to be 0.12. The Néel temperature of a hypothetical compound with fluorite structure in which all the cation sites are occupied by the U 5+ ions, was estimated to be 15 K.