The isotachophoretic separation behaviour of metal ions in a model soluition of a high-level liquid waste (HLLW) was investigated by means of the isotachophoresis—particle-induced X-ray emission (ITP-PIXE) method for the purpose of fractionation of useful elements contained in the HLLW. The leading electrolyte used was 20 m M ammonia solution buffered by acetic acid (pH 4.8), in which a complex-forming agent (10 m M α-hydroxyisobutyric acid) was contained. The migration order of the cationic components was (Rb +, Cs +), NH 4 +, Ba 2+, Sr 2+, Na +, Mn 2+, Fe 2+, (Cr 2+, Rh 2+, Cd 2+), Ni 2+, La 3+, Ce 3+, Pr 3+, Nd 3+, SM 3+, Eu 3+, Gd 3+ and Y 3+, where the ions in parentheses could not be separated from each other. The separation efficiency of rare earth elements was 178 nmol/C and it was decreased to 96 nmol/C when NaNO 3 coexisted as the matrix. Part of the Fe 3+, Zr IVO 2+, Mo IVO 4 2− and platinum group elements formed a cationic colloidal zone. Although the cationic recovery was 100% for most of the components, a substantial part of Fe, Te VI O 3, Zr IVO, Ru, Rh and Pd did not migrate, suggesting the formation of non-ionic hydrolysis species. Although ITP was not suitable for their analysis, platinum group metals in the HLLW might be partitioned efficiently as non-ionic components.