Reported are experiments to evaluate ultrafiltration (reverse osmosis) as a method for removal and analysis of pesticides from water supplies and other aqueous solutions. The selectivity of five types of reverse osmosis membranes was determined for aqueous solutions of several derivatives of 2,4 dichlorophenoxyacetic acid (2,4-D). Large differences were noted in the retention properties of various commercial membranes. Tests performed with laboratory-prepared tri-acetate membranes and with iron complexed forms of 2,4-D were also discouraging. Therefore, it does not seem likely that ultrafiltration can be used effectively as a means of complete removal or analysis of 2,4-D and its derivatives in water supplies, but partial removal of pesticide residues may be a beneficial side effect of water treatment by reverse osmosis, depending on the residue and the membrane used.