Vacuum membrane distillation (VMD) is a separation process based on the use of microporous hydrophobic membranes. The membrane is located between an aqueous phase and a permeate, which is kept under vacuum at pressure values below the equilibrium vapor pressure of the feed. The liquid stream vaporizes at one side of the membrane, and the vapors diffuse through the gas phase inside the membrane pores. The process rate and performance are affected highly by the transport phenomena both in the liquid phase and through the membrane. Heat- and mass-transfer resistance in the liquid phase, as well as mass-transfer resistance through the membrane, play an important role in determining the process performance. Based on VMD experimental data for several binary aqueous mixtures containing volatile organic compounds, a simple criterion to investigate the role of each transport resistance on the separation efficiency is discussed.