Membrane distillation is a membrane process in which two liquid phases at different temperatures are separated by a microporous hydrophobic membrane. The membrane plays the role of physical support for the vapor-liquid interface. Pure water transport through PTFE and PVDF flat membranes was studied, as well as the dependence of the phenomena on average temperature and recirculation rate at membrane sides. The influence of these operating conditions on mass transfer is also discussed taking into account mass and heat transfer within the membrane and adjoining liquid phases. The concept of temperature polarization introduced into the transport equations shows its importance in experimental results.