High-purity cerium dioxide (CeO?) powder serves as a key raw material for chemical mechanical planarization (CMP) slurries, wherein trace impurities can severely degrade its polishing efficiency and surface modification performance. This study established a reliable method for the determination of trace rare earth element (REE) impurities in high-purity (CeO?) via online aerosol dilution inductively coupled plasma mass spectrometry (AD-ICP-MS) combined with solvent extraction pretreatment. Using 2-ethylhexylhydrogen-2-ethylhexylphosphonate (EHEHP) as the extractant, the tetravalent cerium matrix was efficiently separated with a separation efficiency of ~99.8 %, and the recoveries of target REEs ranged from 92 % to 98 %. Furthermore, residual Ce-derived mass interferences on gadolinium (Gd) and terbium (Tb) detection were effectively eliminated by introducing 0.80 L min?1 argon into the sample aerosol before the ICP. Under the optimized conditions, the formation yields of interfering polyatomic ions were suppressed to as low as 0.12 % for CeO?/Ce? and 0.008 % for CeOH?/Ce?. The limits of quantification (LOQs, 10σ) for 14 REE impurities were determined to be 0.2–1.3 ng g?1. The proposed method was successfully applied to the analysis of two high-purity CeO? samples. The satisfactory spike recoveries of 93–103% verified that the proposed approach is robust and promising for the trace analysis of REE impurities in high-purity cerium oxide materials.