Fluorescence spectroscopy is a convenient tool to examine peptide-membrane interactions at equilibrium (1) owing to the change in emission properties of many fluorophores (including tryptophan) during transfer from an aqueous environment into a lipid bilayer. In some cases (e.g. mechanosensitive channel blocker GsMTx4 described here), however, binding-associated emission changes are too small for reliable determination of the free energy of partitioning, ΔG. To enhance the spectroscopic response to binding we implemented the titration with lipid vesicles in the presence of aqueous ionic quencher iodide, which preferentially quenchers fluorescence of the free peptide in solution. We have verified the accuracy of this new titration protocol using the well-studied peptide melittin.