The distribution of vanadium(N) and vanadium(V) between an aqueous solution and the chelating resin Chelex 100 was investigated on the basis of the Gibbs-Donnan model. As expected from this model the thermodynamic equilibrium constant in aqueous solution and in the resin phase were found to be equal. The following values for the complexation constant and for the exchange coefficients in NaNO, 0.1 M were found: log K = -1.93 and log ploeX --1.89 for vanadium(N) and log K 5 0.81 and log pllcX -0.70 for vanadium(V). A complex VO,L (log ploeX = -1.48) was found to be formed inside Chelex 100 at pH higher than 3.5. 'Ihe competition by the hydrolysis reactions in aqueous solution was quantitatively considered to explain the sorption curves of vanadium(V) in the acidity range up to pH 9. As vanadium(W) is not desorbed at pH higher than 8 a separation method was proposed, based on a batch technique. It consists of sorbing both vanadium(V) and vanadium(N) at a pH of about 4.5, and separating them by stripping vanadium(V) at a pH of about 10 and then vanadium(W) at pH about 0.8. A preconcentration factor of 40 could be obtained in this way. The model was also used to predict conditions for separation by a column method. The interference of fulvic acid on the oxidation state of vanadium and on its sorption on Chelex 100 was examined.