A simple and rapid HPLC-ICP-MS method for determination of ultra-trace REE impurities in highly pure Eu₂O₃ and Yb₂O₃ has been developed. The majority of REEs as trace impurities in both Eu₂O₃ and Yb₂O₃ matrices can be determined solely by ICP-MS. However, ¹⁵³Eu¹⁶O⁺ and ¹⁷⁴YbH⁺ formed in the plasma of the ICP-MS overlap with the monoisotopic ¹⁶⁹Tm⁺ and the most abundant isotope of Lu (¹⁷⁵Lu⁺), respectively. Spectral interferences caused by the formation of matrix-based polyatomic ions are eliminated by separating the highly concentrated matrix element from the spectrally interfering elements of interest. This is achieved by coupling HPLC to ICP-MS. A reversed-phase C-18-silica HPLC column impregnated with 2-ethylhexyl, 2-ethylhexyl phosphonic acid (H[EH(EHP)]) is prepared and used as a separation tool. The method allows separation of problematic REEs, viz Eu from Tm and Yb from Lu in time and space prior to entering the ICP-MS, which enables accurate determination of the REEs in question. The method is used to characterize high purity Eu₂O₃ and Yb₂O₃ through the impurity approach, focusing on trace concentrations of other REEs in the respective oxides. The method is rapid with a separation time of 3.5 minutes, which makes it suitable for routine analysis of trace levels of all REE impurities, including Tm and Lu in high-purity europium and ytterbium oxides. The chromatographic system is compatible with ICP-MS as the mobile phase consists only of diluted HNO₃.