Experiments are conducted to compare and contrast the performance of Water Gap Membrane Distillation (WGMD) and Air Gap Membrane Distillation (AGMD) designs under different operating and design variables. The membrane module is designed to work on both designs exchangeably. The effects of feed flow rate, feed temperature, gap width, coolant flow rate, feed concentration, and the material of membrane supporting plate on the permeate flux are investigated experimentally. The temperatures inside the gap are measured to evaluate the changes in performance in relation to the gap temperature. Results showed that the water gap design enhances the permeate flux significantly. The increase is flux ranges between 90% and 140%, mainly depending on the feed temperature, when using the water gap as compared to the air gap. The temperature inside the water gap is lower than that of the air gap under the same operating conditions. Having a liquid interface on the cold side of the membrane increases the evaporation in feed side, maintains lower gap temperature, promotes the condensation process, and thus enhances the flux. Increasing the gap width reduces the flux, particularly at higher temperature. However, the water gap is found to be less sensitive to gap increases compared to air gap. It is recommended to use brass plate for supporting the membrane with the water gap, regardless of the gap width. On the other hand, with the air gap, the material thermal properties become less effective as the gap increases. A clear decrease in the flux is recorded with increases of the feed concentration due to the effect of concentration polarization on the feed side of the membrane. A salt rejection factor up to 99.98% is achieved with both air and water gaps. (C) 2014 Elsevier B.V. All rights reserved.