A twin-head cationic surfactant was used in micellar-enhanced ultrafiltration of aqueous solutions of benzoic acid. The surfactant, a tertiary amine with two polyoxyethylene head groups and an alkyl tail of 18 carbons, had a critical micelle concentration of 0.06 mM (0.05 g/L). Semiequilibrium dialysis measurements gave a maximum solubilization capacity of 1 mole of acid per mole of surfactant. The ultrafiltration was performed in hollow fiber units having molecular weight cutoffs of 5 K and 30 K at transmembrane pressures up to 138 kPa. The permeate flux was independent of the concentration of surfactant at concentrations below 4 g/L and decreased at higher concentrations. The flux was independent of the concentration of benzoic acid. At a fixed benzoic acid concentration, the rejection of acid increased with increasing concentration of surfactant, passed through a maximum, and then decreased. For surfactant concentrations above about 2 mM and below 20 mM, the rejections of the surfactant and benzoic acid were functions only of the ratio of surfactant to solute. The best ratio was 1.2 moles of surfactant per mole of benzoic acid, where the rejections were 0.89 for the acid and 0.95 for the surfactant.