When a microporous hydrophobic membrane separates two aqueous solutions at different temperatures, selective mass transfer across the membrane is obtained. The process can be carried out at the atmospheric pressure and at temperatures which might be much lower than the boiling point of the solution. At the optimal membrane microporosity, polymer hydrophobicity and thermal conductivity, only water vapor is transported in the membrane phase and condenses as liquid on the low temperature side of the membrane. The driving force is the vapor pressure difference between the liquids at the two solution-membrane interfaces. Various hydrophobic polymeric membranes are available, prepared from polypropylene, PVDF and Teflon, in the flat sheet or capillary configuration. The membranes prepared by thermal phase—separation technique are particularly interesting. INTRODUCTION Membrane Distillation is a new, rapidly increasing membrane technology, characterized by the possibility of overcoming some limits of other membrane processes, such as reverse osmosis (ref. 1,2). When compared to traditional evaporation, membrane distillation offers also the basic advantages of membrane separations: easy scaling up, simplicity of operations, possibility of high membrane surface/volume ratio, etc. Moreover, there exists the possibility of treating solutions with thermosensitive compounds and high level of suspended solids, at a temperature much lower than the boiling point and at the atmospheric pressure. Theoretical 100% rejections might be predicted for all electrolyte and non-electrolyte solutes. The possibility to reach a high solute concentration in the feed is of particular interest, considering the limits of RO due to the osmotic pressure increase with concentration. PROCESSMECHANISM When a microporous hydrophobic membrane separates two aqueous solutions at a different temperature, a net pure water flux from the warm side to the cold one is observed. The process can be described by the following steps: water evaporation at the solution-membrane warm interface, transport of the vapor phase through the microporous system, and condensation at the cold membrane-solution interface (see Fig. 1). The driving force for the vapor transport in this process is given by the vapor pressure difference between the two solution-membrane interfaces due to the existing temperature gradient. The hydrophobic properties of the polymeric material prevent the bulk liquid transport of the liquid phase across the membrane. The morphology of the polymeric membrane is, however, a critical parameter of the process. A maximum critical pore size exists at which the liquid penetrates the microporous hydrophobic phase. This value is given by the Kelvin law: P = 2y cos /r where y is the surface tension of the liquid; 0 is the contact angle between the liquid and the membrane; r is the radius of the pore; P is the applied pressure.