Tri-n-butyl phosphate (TBP) is widely used as an extractant in many technological processes. A thermodynamic model of the base extraction system water-tributyl phosphate is of great importance for industry. In this work a three-dimensional shape of the liquid phase Gibbs energy surface was modeled for the entire concentration range from pure water to pure TBP at temperatures from 273.15 K to more than 400 K and a pressure of 101325 Pa. Volumetric properties of the water-TBP solutions were measured at 288.15, 298.15, and 323.15 K. To obtain a thermodynamic description of the system of interest a new model of the excess Gibbs energy, the generalized local composition model (GLCM), was developed. Density of solutions, activity of components, enthalpy of mixing, and liquid liquid equilibria in the water-TBP system were described by the GLCM expression.