The involvement of macromolecules in the formation of biological and other membranes has important implications for structural biology and nanoengineering. Using cetyl polyethylenimines of varying molecular weight and hydrophobicity, it was found that polymer hydrophobicity (mol % cetylation) controlled the nature of the self-assembly, giving micellar (cetyl groups < 23 mol %), vesicular (cetyl groups = 23-42 mol % or cetyl groups = 3-42 mol % with cholesterol), and dense nanoparticle (cetyl groups greater than or equal to 49 mol %) aggregates. Thick (up to 15 nm) membranes due to the polyelectrolyte coating with the amphiphile were observed with low levels of cetylation only, and both dn/dc (indirectly) and vesicle/nanoparticle size (directly) varied linearly with mol % cetylation (r = 0.96-0.99).