In this paper, the equilibrium study on the reactive extraction of 2-methylidenebutanedioic acid (five initial acid concentrations in the range of 0.05 to 0.25 mol.dm(-3)) from aqueous solution by N,N-dioctyloctan-1-amine (TOA, 0.115 and 0.229 mol.dm(-3)) as extractant dissolved in six different diluents [heptane, kerosene, methyl benzene, decane-1-ol, 4-methylpentan-2-one (MIBK), and dichloromethane (DCM)] is carried out. The extraction results are presented in terms of distribution coefficient (K(D)), degree of extraction (E) and loading ratio (Z). The highest value of the K(D) is found to be 32.478 at 0.05 mol.dm(-3) of 2-methylidenebutanedioic acid concentration with 0.229 mol.dm(-3) of TOA in DCM. The extractability of TOA in different diluents in terms of K(D) is found to be in the order of DCM > MIBK >= decan-1-ol > methyl benzene > kerosene > heptane. The mathematical model is proposed and used to estimate the equilibrium constant (K(E)) and stoichiometry (m, n) of the reactive extraction. Based on the stoichiometry, the corresponding individual equilibrium constants (K(11), K(21), and K(12)) for the formation of (1:1), (2:1), and (1:2) acid:TOA complexes, respectively, are also determined. Further, the experimental values of K(D) are correlated using the linear solvation energy relationship (LSER) model which is based on solute solvent-interaction parameters.