The component activity coefficients in systems of tributyl phosphate (TBP), diluents and water are calculated with a theoretical equation based on perturbation theory. Seven diluents were investigated : n-C6H14, n-C7H16, n-C8H18, C6H6, cy-C6H12, CCl4, and CHCl3. Nine segments (H2O, CH3, CH2, CH(ben.), CH2(cy.), CCl4, CHCl3, CH2O and PO) are involved in the study. The segment parameters of H2O, CH3 and CH2 are obtained from PVT data of pure fluids (H2O, n-C4H9OH-n-C8H17OH, and C2H6-n-C27H56). The other segment parameters are regressed from the activity coefficients of part of the binary systems. The component activity coefficients for other binary, ternary and quaternary systems composed of TBP, diluents and water are predicted with these parameters. The ARDs for prediction are less than 10% in general.