Vapor-liquid equilibrium (VLE) data of solutions are necessary for the design of distillation and absorption processes. VLE exhibits various characteristics depending on the type of solution. In the case of nonideal solutions, an azeotropic mixture is formed which cannot be separated by ordinary distillation. The mixture must be separated by adding a third component, called an entrainer, which has the capability of breaking the azeotropic point. In most cases, a volatile component is employed as an entrainer for an azeotropic mixture. However, salt is also effective in breaking the point; this is called the salt effect on VLE. Much has been observed on salt effect, however very few commercial distillation plants use this method. This book aims to cover all reported data found in journals on salt effect on VLE. Prediction methods for VLE at low and high pressures for systems composed of volatile substances are used routinely, However, no method to predict the salt effect on VLE is in use, because salts show entirely different behavior from volatile substances. A method to predict salt effect based on preferential solvation was reported by the author in 1976.30 systems were examined and the formation of preferential solvates between the salt and one of the volatile components was shown. Continuing the work, the formation of preferential solvates for almost all salt effect data has been examined. As a result of this work, it has been found that preferential solvates are formed without exception. In this volume, the preferential solvation numbers determined by least squares method are shown by processing the data of salt effect on VLE.