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For the first time, a mathematical model is presented to validate the hydrophobic/hydrophilic composite membrane concept in membrane distillation (MD). This mathematical model aims at providing the optimal characteristics of both hydrophobic and hydrophilic layers as well as giving the guidelines to prepare highly efficient MD membranes. The model was tested by the experimental data obtained from four laboratory-made and two commercial membranes. A very good agreement between the experimental and theoretical values for the vapour fluxes was obtained. In addition, the effect of morphological characteristics of both hydrophobic top-layer and hydrophilic sub-layer was studied by simulating the permeate flux, when the membranes were used for direct contact membrane distillation (DCMD). Those characteristics include hydrophobic top-layer porosity and hydrophilic sub-layer porosity, thicknesses of the …