Abstract: Based on the extraction equilibrium behavior of formic, acetic, propionic, butyric, valeric, monochloroacetic, dichloroacetic, trichloroacetic, glycolic, glyoxylic and lactic acid in part (Ⅰ) of this paper, mathematical models describing extraction behavior of monocarboxylic acid with trioctylamine (TOA) was set up by using the mass action law, and the apparent extraction equilibrium constants, K₁₁, K₂₁、K₃₁ and K₁₂ were obtained. The calculated TOA loading approached the experimental data. The extraction equilibrium behavior depended on the three physical properties of carboxylic acid or extractant, i.e. acid hydrophobicity, acid acidity and the specific basicity of TOA. The correlation of K₁₁ with these three properties was obtained, and can be used to predict the K₁₁ of acids.