The influence of pH and diluent on the ion-pair solvent extraction of benzene polycarboxylic acids have been investigated for the separation of the coal oxidation products, which are formed by the treatment with alkaline solutions at high temperatures. Although the extent of the solvent extraction of benzoic acid (1BE) with a quaternary ammonium reagent (tri-n-octylmethylammonium chloride) into chloroform and benzene did not change at a very acidic and alkaline solutions, those of 1,2-benzenedicarboxylic acid (12BE) and trimellitic acid (124BE) somewhat decreased at very low pH and very high pH. The magnitudes of the equilibrium constants (K-ex) of 1BE using a different diluent decreased in the order benzene > carbontetrachloride > 1,2-dichloroethane > cyclohexane > hexane > chloroform > 1-octanol and those of 12BE decreased in the order benzene > cyclohexane > carbontetrachloride > hexane > 1,2-dichloroethane > chloroform. The inspection of the correlation between the values of K-ex and several parameters of the diluent implies that the magnitude of K-ex can be described by using the dielectric constant and the solubility parameter of diluent.