The stability constants of the complexes of trivalent lanthanide cations with various dimethoxybenzoates have been determined by the potentiometric titration method. The dimethoxybenzoates studied were 2,3-dimethoxybenzoate, 2,4-dimethoxybenzoate, and 2,5-dimethoxybenzoate. The thermodynamic parameters for the complexation of some lanthanide cations with 2,5-dimethoxybenzoate were measured by enthalpy titration using a titration calorimeter. All complexation measurements were made at 25 degrees C in an aqueous medium of ionic strength 0.5 M NaClO4. The thermodynamic parameters for the complexation are discussed in relation to the electronic effect of the substituent groups within the ligand. It is proposed that the enhanced stability of the dimethoxybenzoate complexes of lanthanide cations is mainly due to the polarization of the electronic charge density from the methoxy groups through the n-system of the phenyl ring to the caboxylate group. C-13 NMR evidence for charge polarization by the lanthanide cation in the complexation is presented. Theoretical calculations of the charge distributions on the carbon atoms of the 2,4-dimethoxybenzoates were also performed.