It is shown that interfacial composition of the liquid-liquid interfaces is equal to the critical composition. The idea has been rigorously proven for binary regular solutions and has been reasonably supported for non-regular multicomponents, making use of the experimental mutual solubility data and the available information on titration, pulsed gradient spin-echo NMR, and differential scanning calorimetry of some W/O and O/W microemulsions. Using this finding, we, moreover, developed a simple equation for prediction of plait point concentration, having known just one tie-line data. The equation suggests a simple consistency test for measured solubility data, which is shown reliable for a large number of ternary systems. A molecular dynamic simulation also has been performed to support the idea. (C) 2010 Elsevier B.V. All rights reserved.