The aim of this work is to propose an approach for the prediction of the kinematic viscosity of neutral organophosphorus extractant with dilutions which were frequently used in the separation of metal ions and inorganic acids, especially in the nuclear fuel reprocessing. The UNIFAC-VISCO group contribution method was applied, and a new group named phosphate was defined. The required structure parameters of the new group were obtained by quantum chemical calculations, including the density functional theory method for optimizing geometries and the polarizable continuum model for molecular surface area and volume calculations. Interaction parameters for the phosphate group have been fitted to the experimental values from literatures. Finally, to provide data for the testing of predictive ability of the proposed approach, kinematic viscosities of binary mixture of tri-n-butyl phosphate with octane were measured over the entire range of compositions at different temperatures.