In this paper we study water transport in membrane distillation using a flat PTFE membrane. Experiments have been carried out with water and aqueous solutions of NaCl as feed. The effects of temperature and concentration polarization on the reduction of vapor pressure differences across the membrane with regard to the vapor pressure differences corresponding to the bulk phases which are separated by the membrane have been evaluated. A coefficient which measures this reduction has been introduced. This coefficient and the known temperature polarization coefficient coincide when water is used as feed, but they are more and more different when the salt concentration of feed increases. In this last case it is the polarization coefficient introduced here that measures the reduction of flux through the effect of the polarization phenomena. The measured flux results and the calculated polarization results are discussed for different temperatures, recirculation rates, and feed concentrations.