A porous hydrophobic/hydrophilic composite membrane can be prepared by the phase inversion method in one casting step from a polymer solution containing a hydrophilic polymer and a fluorinated surface modifying macromolecule (SMM). The surface analysis of these membranes indicate that there is a fluorine concentration gradient along the membrane thickness resulting from the migration of fluorinated blocks to the air side surface during membrane formation. In this paper, direct contact membrane distillation (DCMD) process has been applied, together with a new theoretical model, to estimate the hydrophobic-layer thickness of porous composite membranes. The theoretical model involves the structural characteristics of the membranes, the heat transfer mechanism and the physical nature of mass transport through porous composite membranes. The model was validated using two supported commercial poly-tetra-fluoro-ethylene membranes of different pore sizes and known thickness-layers. The effect of the hydrophilic polymer concentration in the casting solution on the formed hydrophobic-layer thickness of the composite membranes has been investigated. Also are studied the effects of stirring rate and temperature in the DCMD system on the determination of the hydrophobic-layer thickness. The results show that the hydrophobic-layer thickness increases with the concentration of the hydrophilic base polymer in the casting solution and is independent on the operational parameters selected to conduct the DCMD experiments. The proposed method can be used to measure the layer thickness of porous composite hydrophobic/hydrophilic membranes within random uncertainties less than 5%. The maximum deviation of each individual thickness from the corresponding average value is less than 9%. The estimated errors due to the different input membrane parameters associated in the mathematical model were discussed. The hydrophobic-layer thickness of the SMM blended membrane was found to be an order of magnitude lower than that of the commercial membranes frequently used in membrane distillation. (c) 2005 Elsevier B.V. All rights reserved.