Extraction equilibria for saturated aliphatic dicarboxylic acids, namely oxalic, malonic, succinic, and adipic acids, with trioctylamine (TOA) in 1-octanol were determined at various TOA concentrations. Using quantitative FT-IR spectra, we determined that the formation of 1:2 acid-amine complexes depends on the pK(a2) value, and wavenumbers of specific peaks for the COO- of the acid amine ion-pair complexes depend on the pK(a1) value. 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, K-12, and K-21). The extraction abilities for dicarboxylic acids depend on the pK(a1) value. The typical overloading curves of TOA/1-octanol for dicarboxylic acids are given. The loadings of TOA calculated using the equilibrium model parameters and apparent extraction equilibrium constants agree with the experimental data. The apparent extraction equilibrium constants depend on the acidity of the dicarboxylic acid and the specific basicity of TOA. The quantitative correlation of log K-11 (or log K-12) is obtained using pK(a1), (or pK(a2)) and pK(a,B)'.