Unlike liposomes, polymersomes are more colloidally stable, robust, have a low permeability and can be functionalised to tune their properties. Polymersomes have been used for the encapsulation of molecules such as drugs, enzymes, and DNA or RNA fragments and for protein reconstitution, despite their difference in material properties from native membranes such as membrane fluidity and membrane thickness. However, polymer membranes are not an ideal solution for every application: the inherent biocompatibility and biofunctionality of liposomes can be lacking in polymer vesicles. This is especially important for applications requiring protein reconstitution as specific lipid interactions are essential for many membrane proteins to retain functionality. Combining lipids and polymers into a hybrid vesicle is one potential solution to the challenges that arise from using pure systems and membrane proteins have been successfully reconstituted within them. Due to their technological interest, numerous studies report on the physical properties of hybrid vesicles. . Previous studies have shown that 1-palmitoyl-2-oleoyl-snglycero-3-phosphocholine (POPC) and poly(1,2-butadiene-block-ethylene oxide) (PBd-PEO) form well-mixed hybrid vesicles with no evidence of compositional heterogeneities on optical length scales. The favourable mixing of these components has been attributed to the similar Hildebrand solubility parameter of the lipid and amphiphilic block copolymer components, i.e., here PBd mimics the long acyl chains of a phospholipid, while PEO is hydrophilic like the phosphocholine head group. PEO is also known to improve the pharmacokinetic properties of vesicles due to its strong interaction with the hydrating water layer that imparts a stealth effect by suppressing binding of opsonin proteins to its surface. This hinders the recognition of the vesicle as a foreign body by the immune system. Hybrid vesicles containing the lower molecular weight PBd-PEO polymer are more likely to form hybrid vesicles. Other studies have used PBd-PEO of different molecular weights to form membranes suitable for membrane protein reconstitution. Understanding the relationship between structure and material properties is required for the informed design of vesicular properties, including properties required for successful reconstitution of membrane proteins. However, there are currently limited high resolution structures of these hybrid membranes. Here, we have performed an in-depth structural characterisation of hybrid vesicles using cryo-transmission electron microscopy (cryo-TEM), including single particle analysis (SPA), cryo-electron tomograph (cryo-ET) and small-angle X-ray scattering (SAXS). This structural information of hybrid vesicles of POPC mixed with PBd22-PEO14 (Ms = 1800 gmol -1 , > 85% 1,2 addition) is used to determine how increasing polymer mole fraction affects the structure and membrane properties of the hybrid vesicles.