GUVs composed of mixtures of the lipid POPC with either PBd22-PEO14 or PBd12-PEO11 were created by using the electroformation method. Hybrid vesicles with 25 mol%, 50 mol% and 75 mol% polymer were studied by confocal fluorescence microscopy. Fluorescent lipid and polymer probes were included within these vesicle mixtures for imaging (2 mol% of the lipid-like 3,3'dioctadecyloxacarbocyanine perchlorate (DiO) dye and 10 mol% of tetramethyl rhodamine labelled polymer, PBd22-PEO14 -TMR). GUVs were successfully formed at all compositions; the majority of GUVs were unilamellar, with some multilamellar and intralumenal vesicles also observed. Quantitative analysis of GUV populations focused on the unilamellar GUVs. At all hybrid compositions, the vesicle membranes appeared homogenous with no phase separation evident due to preferential partitioning of the lipid or polymer probes into domains of distinct composition (Figure ). This is consistent with the observation of homogeneous PBd-PEO/POPC hybrid vesicles across a range of polymer lengths in previous studies. While blending POPC lipid with either PBd22-PEO14 or PBd12-PEO11 polymers might be anticipated to form homogenous hybrid membranes, considering the similar Hildebrand solubilities of the hydrophobic blocks, other energetic factors such as hydrophobic mismatch due to differences in preferred membrane thickness, or kinetic factors in the assembly pathway of the vesicles, could lead to compositional heterogeneities in these samples. To investigate this and verify that hybrid vesicles are indeed formed, the relative intensities of a fluorescently labelled lipid (DiO) and polymer (PBd22-PEO14 -TMR) were measured for each GUV. In Figure , a normalised relative intensity ratio is measured for these two probes, where a value tending towards 1 indicates a higher relative polymer dye intensity within the membrane and hence a polymer-rich vesicle, while values tending towards 1 indicate a higher relative lipid dye intensity, implying a lipid-rich membrane. A relative intensity ratio of 0 indicates the ideal mixing of the lipids and polymers in hybrid vesicles at the expected molar ratio for that sample (see the Experimental section for a more detailed description of this analysis).