The excess molar volumes VEm for ternary pseudo-binary mixtures of [(benzene+chloroform or carbon tetrachloride)+(propionitrile+chloroform or carbon tetrachloride)] have been determined over the full composition range, by measuring the densities with a vibrating-tube densimeter at 298.15 K and atmospheric pressure. The experimental and calculated quantities are compared to those for the binary mixture (benzene+propionitrile), and the mixing behavior of the components is discussed. The results show that adding the third component, chloroform or carbon tetrachloride, to the binary mixture has a small effect on the interaction between benzene and propionitrile.