Previous studies in our laboratory showed that the interaction of peptides with membrane lipids depend on the secondary structure of the former and the physical state of the latter (1, 2). Here, we contribute to this subject by imaging of Langmuir monolayers using the Brewster Angle Microscopy technique (BAM). Monolayer isotherms of the well-known Melittin did not present compressional hysteresis, in keeping with a prevalence of ¥á-helix structure.Melittin form homogeneous monolayers at BAM, butwhen mixed with a lipid that forms Liquid Expanded (LE) monolayers (T_c,dmPC@21¨¬C) it induced domain formation (phase segregation) at lateral pressures that dependent on film composition. Although pH affectsthe isotherm of pure Melittin, this did not affect the phase diagram of Melittin/DMPC. On the contrary, when mixed with the more liquidPOPC (T_c,poPC@ -2¨¬C), the domains were not observed. On the other hand, the ¥â-sheet A¥â1-42 peptide exhibits a different behaviour when mixed with either DMPC or POPC. In A¥â1-42/DMPC mixtures, although phase segregation did not occur at a defined lateral pressure, a fibrillar domain pattern became apparent upon compression. In A¥â1-42/POPC mixtures no domains were observed.We conclude that the physical state of lipid phase is not only important for lipid-peptide miscibility but also for lateral topography.