The extraction of organic pollutants from one dilute aqueous solution to another dilute aqueous solution through silicone rubber membranes has been investigated. Overall mass transfer coefficients have been measured using a single membrane tube held in a glass module, and have been described using a resistances in series model. This has allowed the relative contributions of the liquid film boundary layer resistances and the membrane resistance to be elucidated, and has shown that both liquid film resistances and permeation through the membrane itself can limit the overall extraction rate. Under some conditions the membrane resistance can become negligible relative to the liquid film resistances, An important parameter for determining the magnitude of the membrane resistance is the membrane/aqueous phase partition coefficient, K. This parameter can be estimated from the Hildebrand solubility parameter of an organic compound, or quickly ascertained by performing a set of simple adsorption experiments.