Cerium dioxide and ceria-based materials are widely used as components of polishing mixtures and abrasives, anticorrosive coatings, catalysts, solid-oxide fuel cells, etc. [1, 2]. The interest in cerium dioxide research has considerably increased over the last decade because the transition of this compound to a nanocrystalline state significantly changes its physicochemical properties. In particular, with a decrease in the particle size, the unit cell parameter and the oxygen nonstoichiometry of cerium dioxide increase, its electron and ion conductivities change, and its catalytic and biological activities are enhanced [3‐8]. Current cerium dioxide production methods, including thermal decomposition of cerium(III) and (IV) salts, chemical vapor deposition, microemulsion synthesis, aerosol pyrolysis, etc., usually either fail to give CeO 2 nanoparticles of controlled size, or require expensive reagents and equipment, thus preventing practical application. Therefore, in this work, we propose a new method for synthesizing loosely aggregated well-crystallized CeO 2 nanoparticles of controlled size by hydrothermal microwave treatment of ion-exchanged sols. To obtain the initial hydrated cerium dioxide sols, Amberlite IRA 410 CL ion-exchange resin in the OH form was gradually added to a 0.013 M Ce(NO 3 ) 3 solution until pH 10.0. The resulting stable sols were filtered off from the resin, placed in 40%-filled 100-mL polytetrafluoroethylene autoclaves, and subjected to hydrothermal microwave treatment in a Berghof MWS-3* microwave digestion system at 120 and 180 ° C for 2 h. After the treatment, the autoclaves were cooled in air. The products were centrifuged off, repeatedly washed with distilled water, and dried for 2 h in air at 60 ° C. Reference experiments were carried out under similar conditions, with the initial suspensions being hydrated cerium dioxide precursors precipitated from Ce(NO 3 ) 3 solutions by aqueous ammonia. In some experiments, not only the experimental temperature but also the experiment duration was varied (to 6 h). Cerium dioxide synthesis under ordinary hydrothermal conditions was performed in a Parr 4793 reactor with a polytetrafluoroethylene insert equipped with a Parr 4836 temp controller. X-ray diffraction phase analysis (XRD) of the solidphase synthesis products was performed with a Rigaku D/MAX 2500 rotating-anode diffractometer ( Cu K α radiation). The coherent scattering domain sizes D CSD were calculated by the Selyakov‐Scherrer formula