The thermodynamic parameters of formation of 1:1 complexes between lanthanide cations and some benzoic acid derivatives were determined by potentiometric and calorimetric titration methods in aqueous solution. The benzoic acid derivatives studied were 4-aminobenzoate, 4-hydoxybenzoate, and 4-nitrobenzoate. All complexation measurements were made at 25 degrees C in an aqueous medium of ionic strength 0.1 M NaClO4. The thermodynamic parameters for the complexation are discussed in relation to the electronic effect of the substituent groups within the ligands. It has been found that the stabilities of the complexes are affected by the nature of the para-substituted group in the phenyl ring of the carboxylate ligands. Theoretical calculations of the charge distributions on the carbon atoms of various substituted benzoate ligands were also performed. It was found that the stability constants of the complexes increase with increasing negative charge shifts of carboxylate carbon atoms of substituted benzoate ligands in changing from the ionized form to the metal-complexed form.