Self-assembly of lipids and block copolymers into mixed giant unilamellar vesicles (GUVs) and the underlying phase-behavior are reported, based on the well-known assembly of natural lipids and synthetic di- and triblock copolymers in dilute solutions. In this publication the formation of mixed vesicles containing DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and newly prepared PIB–PEO diblock copolymers (BCP) was investigated viaconfocal laser scanning microscopy and Langmuir films. Polyisobutylene (PIB) and poly(ethylene oxide) (PEO) are highly biocompatible polymers and their incorporation into DPPC vesicles therefore leads to interesting structural modifications upon the formation of hybrid GUVs. Results of the confocal microscopy studies prove the mixing/demixing behavior of lipid and polymer molecules in the corresponding bilayer membranes. In particular a homogenously mixed phase below 20 mol% and above 30 mol% of PIB₈₇-b-PEO₁₇, but also the formation of demixed phases of DPPC with 20 to 28 mol% of BCP were observed. Lipid/polymer interactions in mixed monolayers were further investigated by Langmuir balance techniques coupled with fluorescence microscopy. It is shown that the typical behavior of DPPC monolayers at the air/water interface is strongly disturbed by the presence of the diblock copolymers, further proving phase separation.