The objective of this study was to analyze the polarization phenomena in each side of a microporous hydrophobic membrane using a direct contact membrane distillation process. Experiments were conducted with distilled water and sodium chloride aqueous solutions as feeds. Different flow rates and temperatures at both membrane sides were employed. The feed and permeate temperature polarization coefficients as well its the feed and permeate vapor pressure polarization coefficients were defined and evaluated. Two methods: a velocity extrapolation method and a semiempirical method that considers the heat and mass transfer empirical correlations, were used. It was proved that there is an asymmetric polarization in direct contact membrane distillation.