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The interaction of V(IV) and V(V) with succinic acid was investigated by affinity capillary electrophore- sis (ACE) in aqueous acid solutions at pH values 1.5, 2.0 and 2.4, and different ligand concentra- tions. V(IV) and V(V) form protonated complexes with succinic acid ligand at this pH range. The loga- rithms of the stability constants, measured at 0.1 mol L-1 (NaClO4 /HClO4) ionic strength and 25 °C, are logβ111 =7.4 ± 0.2 and logβ122 =14.1 ± 0.5 for V(IV), and logβ111=7.3 ± 0.1 for V(V), respectively. The stability constant values, extrapolated to zero ionic strength with the Davies equation, are logβ°111=8.3 ± 0.2 and logβ°122 =15.6 ± 0.5 for V(IV) and logβ°111 =7.9 ± 0.1 for V(V). The application of ACE to the simultaneous equilibria of V(IV) and V(V) (injection of two analytes) was also attempted. When the results were compared with those obtained when introducing only one analyte in the capillary, using the traditional version of the method, similar stability constants and precision are obtained. The possibility of studying two analytes simultaneously decreases the time needed for the determination of the constants ; this feature is especially valuable when working with hazardous materials or when only small quantities of ligand are available.