The peak intensity values of individual vesicles were further used to determine, whether there was a transmembrane asymmetry in the different leaflets of the membrane (method and data available in SI Section 3). Consistently significant differences in electron intensity between the inner and outer peaks would be indicative of membrane asymmetry. However, this fractional difference (relative difference between electron density) was less than 5% for all vesicle populations, including those composed of pure lipid or pure polymer, suggesting that none of the hybrid membranes had significant transverse compositional asymmetry. We attribute the slight electron density asymmetry that is observed in all vesicles to the curvature of the membrane, where the inner leaflet is slightly compressed when compared to that of the outer leaflet.