We have previously shown that peptides such as G(IIKK)4I-NH2 (G4) can kill bacteria and inhibit cancer cell growth whilst remaining benign to mammalian host cells. We have also demonstrated using Langmuir trough measurements that these peptides can selectively respond to different membrane types and that charge interaction is a crucial factor . Here we propose to use neutron reflection to study how these peptide interact with lipid monolayers under different surface pressures. As peptide binding follows a dynamic process over a period of 30-60 min during which the surface pressure rises steadily, we will plan measurements under different contrasts with a carefully select set of pressure and time windows.