This model considers both specific chemical and nonspecific physical contributions to excess Gibbs energy. Competitive chemical equilibria are evaluated to describe specific chemical interactions. Molecular size, shape difference, and regulation solution terms, based on the "true" species, are used to describe nonspecific physical interactions. Parameter estimates are obtained by fitting vapor-liquid equilibrium and hydrogen and phosphorus NMR shift data for three tri-n-butyl phosphate (TBP) and diluent binary mixtures. The thermodynamic equilibrium constant for the TBP dimer is a shared parameter since this complex apparently exists in all binary mixtures. At 22 degrees C, its value is 6.65 based on the behavior of TBP diluted with either benzene, chloroform, or n-heptane. Thermodynamic equilibrium constants for the 1:1 TBP/chloroform and TBP/benzene complexes are 37.88 and 1.34, respectively. The 1:2 chloroform/TBP dimer complex has an equilibrium constant of 37.57. Model parameters are used to predict excess molar volumes and enthalpies. Commonly reported stability constants are shown to be concentration dependent.