Water processing method, in particular for producing fresh water from salt water by membrane distillation. In comparison with previously known methods, a significant reduction in investment cost and operating cost can be achieved by the combination of the following measures: The water to be processed is kept in a supply chamber the wall of which is formed at least in part by a hydrophobic membrane being permeable for water vapor. A hydrophilic membrane having a greater thickness in comparison with the hydrophobic membrane and a lower thermal conductivity per unit area runs in parallel with the hydrophobic membrane. By the pumping action a vapor pressure difference is produced between the water to be processed and the fresh water so that the membrane distillation is driven by the vapor pressure difference resulting from the pumping action, the water condensing in the pores of the hydrophilic membrane.