The desired characteristics needed for a membrane to be used in direct contact membrane distillation (DCMD) are outlined. A new approach for the design of novel membranes for desalination by DCMD is proposed. Porous hydrophobic/hydrophilic composite membranes have been prepared in one casting step, using the traditional phase inversion method of polymer solutions containing a hydrophilic host polymer and a fluorinated surface modifying macromolecule. The membranes comply with the conditions and properties required in DCMD process. Membranes of different mean pore sizes, pore size distributions, porosities, roughnesses, liquid entry pressures of water and thicknesses can be formed by varying the membrane preparation conditions. The structural characteristics and the DCMD performance of the first proposed porous composite membranes were compared with those of the commercial membranes most frequently used in this kind of process. The proposed membranes have potential to be used in DCMD for desalination because, compared to the commercial ones, these membranes possess a thinner hydrophobic porous top-layer responsible for the water vapour transport in DCMD and a thicker hydrophilic porous sub-layer filled with water to lower the conductive heat loss. The thickness of the hydrophobic layer was found to be an order of magnitude lower than that of the commercial membranes.