The limiting organic phase concentration (LOC) above which a third phase forms, in Thorium Nitrate - 30% TBP/Diluent system has been shown to have a good correlation with some parameters which characterise the diluent such as Connectivity Index, Wiener Number and Diluent Parameter. Aliphatic, alicyclic, aromatic and halocarbon type diluents have been used in this study. The LOC has also been found to be additive for mixtures of diluents. The results make it possible to predict third phase formation in systems where the diluent composition is known.