Concentration polarization affects almost all membrane processes - it reduces the available driving force; thus the flux of the more permeable species is reduced and the leakage of the less permeable species is increased. For membrane separation in hollow fiber modules, the concentration polarization of the solute species occurs in the mass boundary layer, which is primarily caused by the hydrodynamics. A general theory relating to the hydrodynamic parameters, mass transfer coefficient, and concentration polarization index is proposed for mass transfer in a hollow fiber module. The present analysis offers a clear division between the hydrodynamic part and the pure membrane contribution in the membrane separation processes of liquid phases. The effect of flow dynamics on polarization is also discussed. The experimental data of pervaporation and dissolved gases offer good correlations with the theory, which will help the membrane design and improve operation efficiency.