Polymeric membranes are currently adopted in water purification processes, such as reverse osmosis (RO) and membrane distillation (MD). This latter technique is very promising for separation effectiveness and energy savings. A valuable and effective MD unit must be equipped with polymeric membranes that exhibit specific properties, for example, hydrophobicity, a narrow pore size range, a high water penetration pressure, and a large vapor permeability. In this work, we present and examine the main features of membranes for MD processes, with the aim of experimentally evaluating the related performances. Scanning electron microscopy analysis was carried out for a first estimate of the pore size distribution; this showed a discrepancy between the nominal pore size and the actual distribution. Vapor flux under typical water purification conditions was measured for different commercial membranes, and values as high as 35 kg m(-2) h(-1) were observed. Polytetrafluoroethylene membranes were more permeable for both of the MD configurations tested. The liquid entry pressure (LEP) showed an irreversible decrease in the early operation cycles related to microstructural modifications. A LEP reversible decrease with increasing temperature was observed; this showed that in the membrane selection, the nominal LEP value had to be corrected for the temperature effect for the correct choice. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 3557-3563, 2011