Membrane distillation is an emerging technology for treating highly impaired wastewaters. Here a number of commercial membranes have been tested. Bulk membrane and surface properties have been determined. Permeate flux has been determined using model 20,000 ppm NaCl feed streams. A bulk membrane structural parameter has been defined. The structural parameter is used to identify membranes that display the highest permeate fluxes. These membranes were tested with 100,000 ppm NaCl solutions. The maximum feed concentration was determined. For model low-fouling feed streams, membrane surface properties such as hydrophobicity and roughness have less effect on permeate flux than bulk membrane properties.