Liquid-liquid equilibria for aqueous solutions of carboxylic acids, namely formic, acetic, propionic, butyric, valeric, monochloroacetic, dichloroacetic, trichloroacetic, glyoxylic, glycolic, and lactic acids, with trioctylamine (TOA) in various diluents, including 1-octanol, chloroform, methyl isobutyl ketone (MIBK), tetrachloromethane, and hexane, were determined at various TOA concentrations. The loading of TOA for a given carboxylic acid depends on the nature of the solute; the hydrophobicity and pK(a) of the acid; and the nature of solvent, diluent type, and TOA concentration, which can be expressed by the specific basicity of TOA (pK(a,B)), and increases with increasing acidity and hydrophobicity of the acid. The strength of solvation of the complex by the diluent for the acids studied decreases in the order 1-octanol greater than or equal to chloroform > MIBK > tetrachloromethane > hexane. An equilibrium model is presented that employs the mass action law and is used to determine model parameters and apparent extraction equilibrium constants (K-11). The apparent extraction equilibrium constants depend on the hydrophobicity and acidity of the carboxylic acid, as well as the specific basicity of TOA, and the quantitative correlation of log K-11 is obtained using log P, pK(a), and PK'(a,B).