The disruption of membranes by antimicrobial peptides is a multi-state process involving significant structural changes in the phospholipid bilayer. However, direct measurement of these membrane structural changes is lacking. We used a combination of dual polarisation interferometry (DPI), surface plasmon resonance spectroscopy (SPR) and atomic force microscopy (AFM) to measure the real-time changes in membrane structure through the measurement of birefringence during the binding of magainin 2 (Mag2) and a highly potent analogue in which Ser(8), Gly(13) and Gly(18) has been replaced with alanine (Mag-A). We show that the membrane bilayer undergoes a series of structural changes upon peptide binding before a critical threshold concentration is reached which triggers a significant membrane disturbance. We also propose a detailed model for antimicrobial peptide action as a function of the degree of bilayer disruption to provide an unprecedented in-depth understanding of the membrane lysis in terms of the interconversion of different membrane conformational states in which there is a balance between recovery and lysis.