In the current study, we examined the effect of polymer characteristics on the structure of complexes formed between poly (methacrylic acid-co-n-alkyl methacrylate) and with phosphatidylcholine/cholesterol liposomes. We varied the polymer concentration in the vesicles, the preparation concentration of lipid and polymer components during preparation, the molecular weight of the polymer chain, the molecular weight of the polymer's hydrophobic side groups and their mole fraction. The vesicle behavior indicated polymer-free bilayers and bilayers complexed with polymer coexisted at low polymer concentrations. As the polymer concentration exceeds a critical level, however, the system became homogeneous, indicating bilayer uniformity of the bilayer. As the polymer content was raised, the vesicle size and fluidity increased, and the transition temperature decreased. We found that the vesicle size mostly affects the membrane fluidity. We also found that the thermal properties (transition temperature and the magnitude of heat capacity of the peak, Delta Cp) are governed by the effects of the polymer on the structure of bilayer. The length of the alkyl chain of the polymer is shown to significantly affect the structure of polymer-liposome, complexes, as did the chain molecular weight and mole concentration of hydrophobic group in the polymer. (C) 2007 Elsevier Inc. All rights reserved.