Abstract: On the basis of literature survey, trialkyl phosphine oxide(TRPO) was selected as the extractant. Nine kinds of mono-carboxylic acids, formic, acetic, propionic, butyric, caproic, monochloroacetic, dichloroacetic, trichloroacetic and lactic acids, were used as solutes to be extracted. Kerosene was investigated for their abilities to affect the relative basicity of extractant (pK_a,BS). The dependence of TRPO concentration, hydrophobicity and pK_a of the acid on pK_a,BS was studied in order to explore the feasibility of mathematical models based on the chemical reactive theory to describe the extractive equilibrium behavior with only two parameters, hydrophobicity and pKa,that affect pK_a,BS.The pK_a,BS value hardly changed with TRPO concentration, while the properties of carboxylic acid, hydrophobicity and pK_a affect pK_a,BS obviously.The mathematic models proposed in this paper could be used to predict the extractive equilibrium of monocarboxylic acid extracted by TRPO.A simple and feasible modification method for measuring pK_a,BS was also suggested.