Viscosities and densities of the ternary liquid mixtures of tri-n-butyl phosphate + benzene + toluene were measured at (30, 35, 40, and 45) degrees C. The deviations in the viscosity from a mole fraction average were fitted in a Redlich-Kister type equation, which includes the contribution of each constituent binary system along with a ternary contribution term. The viscosity deviations were negative in most of the cases.