In this work, the solvent extraction of vanadium by using the primary amine N1923 as extractant was studied. Within different vanadium concentration ranges, the extraction reactions were determined using the slope method at T = 298.15 K based on the vanadium phase diagram. The slopes (the stoichiometric ratio) were determined as 5/3, 1 and 11/3 for (RNH2)(5/3) H3VO4; (RNH2)H3V3O9 and (RNH2)(11/3)H6V10O28, respectively. The thermodynamic model was established through regression of the experimental results with the Pitzer equation for both aqueous and organic phases by the General Algebraic Modelling System (GAMS). Three different sets of parameters and chemical equilibrium constants K were determined, which corresponded to the three extraction reactions. The orders of the lnK values are V-10 > V-3 > V-1 (organic V species), and the distribution coefficients (D-V) are D-V1 > D-V3 > D-V10. The new model was applied to predict the regularities of extraction yield along with different initial conditions, and the results agree well with the experimental values. (C) 2014 Elsevier Ltd. All rights reserved.