Surfactant incorporation into bilayer vesicle membranes made of diblock copolymers was examined for the case of weak affinity between the membrane [poly(ethyl ethylene)-co-poly(ethylene oxide)] and the surfactant [Pluronic L31]. For copolymer vesicles formed in the absence of surfactant and subsequently exposed to surfactant solutions, a micropipet aspiration technique was employed to monitor surfactant incorporation kinetics and to characterize surfactant-induced changes in the membranes. Though the surfactant incorporation was weak and reversible, it reduced the area expansion modulus by almost a factor of 2, while dramatically increasing the vesicles' susceptibility to lysis. Additionally, water permeability was increased by a factor of 2. The surfactant incorporation rates were proportional to the free surfactant concentration, and other features of the membrane response to surfactant exposure were consistent with transport-limited surfactant attack of the membranes. The most plausible mechanism for surfactant interaction with the membrane is that the surfactant resides at the interface between the hydrophobic membrane core and the poly(ethylene oxide) corona, reducing the interfacial tension of the lamella and allowing the membrane to thin slightly, also increasing its area per unit mass. Observations suggest slow surfactant penetration of the hydrophobic core such that this particular surfactant acts on both inner and outer leaflets.