We determined how the structure and interactions of liposome–peptide complexes is modulated by membrane composition and solution conditions. An intestine-penetrating peptide was synthesized in the lab of C. Gilon. The peptide contained a single charged amino acid in the center and two short hydrophobic moieties. We found that this peptide can strongly couple like-charged membranes. The coupling is much stronger than the effect of screening owing to the equivalent concentration of added monovalent salt. Before the peptide strongly couples the membranes, it goes into the membrane, reduces its thickness and reduces the repulsive interactions between the membranes. The strong coupling occurs up to a critical membrane charge density at which the electrostatic repulsion overcomes the hydrophobic attraction. The peptide penetrates more readily into bilayers that contain higher molar fraction of DOPE owing to steric effects and/or hydrogen bond formation. The peptide can also change the spontaneous curvature of membranes: membranes that form inverted hexagonal phase undergo a phase transition into lamellar phase under suitable conditions (Soft Matter 2013).