The effects of acetate (Ac) concentration on the separation of lanthanides from hydronium ion (H3O+) in capillary zone electrophoresis is investigated. The effects of sample acidity and buffers on the H3O+peak have been studied. A mixture of 14 rare earth metals, along with H3O+, can be baseline separated in less than nine minutes by use of a ternary buffer system [sodium acetate (NaAc) /α-hydroxyisobutyric acid (HIBA) / UV CAT-1]. In contrast, replacing NaAc by equal molar sodium chloride (NaCl) in the buffer decreases the electroosmotic flow without enhancing selectivity and resolution of lanthanides. All of these results suggest that the higher mobility H3O+ion elutes slower than would be predicted. The late appearance of H3O+at pH=4.4 is attributed to the weak acid-base equilibria of HIBA and HAc in the buffers as H3O+migrates through the capillary. The improved separation of metal ions from H3O+by the addition of sodium acetate is also due to the equilibrium of the weak acid (HAc).