The aim of this study was to analyze the transport of water vapor by membrane distillation through a polytetrafluoroethylene flat membrane. Experiments were carried out for different salt concentrations in the feed, average temperatures, and recirculation rates at membrane sides. The influence of these operating conditions on water-vapor flux is discussed, taking into account mass and heat transfers within the membrane and the adjoining liquid phases. The polarization layers formed on either side of the membrane prove to be important in the interpretation of experimental results.