A similar phenomenon is observed for PBd12-PEO11 hybrid GUVs in Figure . The shorter PBd12-PEO11 polymer is closer to the lipids in Mw but still a broad monomodal distribution is observed centred on an evenly mixed GUV composition. Of note, 50 mol% PBd12-PEO11 hybrid GUVs exhibited the broadest distribution in relative intensity ratio, with a broader tail towards polymer-rich compositions. This suggests that near equimolar lipid-polymer hybrid GUVs might be less favourable to form, when compared to the lipid-rich 25 mol% and polymer-rich 75 mol% compositions. This is analogous to early work on POPC/PBd46-PEO30 hybrid vesicles, where GUVs of intermediate lipid/polymer composition ratios could not be formed by electroformation. However, there is no evidence of enhanced heterogeneity in 50 mol% hybrid GUVs formed with the intermediate size PBd22-PEO14 polymer (Figure ). As the two fluorophores present in the hybrid GUVs have overlapping emission and excitation spectra, FRET from the lipid fluorophore, DiO, to the polymer PBd22-PEO14-TMR probe could occur. This would decrease the fluorescence intensity from the lipid DiO, potentially shifting the relative intensity ratio towards 1 and resulting in the observation of apparently polymer-rich membranes. This effect is not seen in Figure , likely due to a low FRET efficiency and the normalisation of DiO intensity in the analysis of these images. Sequential imaging of the two fluorophores means that FRET-enhanced fluorescence of the PBd22-PEO14-TMR probe would not be observed. Therefore our experimental design negates any potential effects from FRET between these probes that could impact our results and interpretation.