Abstract: Mass transport across the membrane must be discussed fundamentally on the basis of the thermodynamics because the thermodynamics describes the rule of energy changes inevitably generating in the mass transport. The classical thermodynamics have introduced a lot of information on the basis of the first and the second law. However, we have to notice that the classical thermodynamics discusses only reversible phenomena and it does not treats the transport rate, in other words, it does not includes the parameter of “time”. Many phenomena (including membrane phenomena) generating in the natural world are far away from reversible states, so the reversible thermodynamics was ineffective to discuss their mechanism. On other hand, the irreversible thermodynamics (Non-equilibrium thermo-dynamics) came to succeed in discussing the mass transport by introducing the concept of “time” in its system. The irreversible thermodynamics is not general theory applicable to analyze every reaction in the natural world. Further it should be noticed that the applicable limit of the irreversible thermodynamics must be determined through experiments. In this chapter, we discuss the irreversible thermodynamics keeping the electrodialysis in mind and conceptually explain the mechanism of saline water desalination in Sections 5 and 6.