- Studies on aqueous vesicle-polymer systems are a challenging area of colloid science, both from an experimental1-5 and a theoretical6 viewpoint. Vesicle-polymer systems constitute good model systems for living cells,7 and they may also lead to important phase behavior phenomena, such as the formation of gels and networks,8-10 with potential industrial applications. Polymers can also act as liposome stabilizers and controllers of membrane permeability in liposomal systems for drug delivery.11 A first step in the understanding of the molecular interactions in these complex systems is to gain direct microstructural information. In this work, cryogenic transmission electron microscopy (cryo-TEM) is used as an imaging method of vesicle-polymer mixtures, in two regimes of polymer concentration c (in wt %), defined with respect to c*, the critical association concentration: (i) at very low concentration (c , c*), where the polymer was seen to induce dramatic bilayer structural changes; (ii) at high concentration (c > c*), where gel formation occurs. The polymers used were the cationic polyelectrolytes JR400 and LM200 (hydrophobically modified polymer). The vesicles are composed of mixed anionic SDS (sodium dodecyl sulfate) and cationic DDAB (didodecyldimethylammonium bromide) surfactants at a fixed amount of SDS excess, thus bearing a net negative surface charge. These catanionic vesicles are particularly suitable for our goal, since they form spontaneously, remain stable with time, and are essentially small and unilamellar.12 The polymers were initially expected to interact strongly with the vesicles, due to electrostatic and hydrophobic interactions (for LM200).