The exchange broadening of backbone amide proton resonances of a 23-mer fusion peptide of the transmembrane subunit of HIV-1 envelope glycoprotein gp41, gp41-FP, was investigated at ph 5 and 7 at room temperature in perdeuterated sodiuxsavem dodecyl sulfate (SDS) micellar solution. Comparison of resonance peaks for these phs revealed an insignificant change in exchange rate between ph 5 and 7 for amide protons of residues 4 through 14, while the exchange rate increase at neutral ph was more prominent for amide protons of the remaining residues, with peaks from some protons becoming undetectable. The relative insensitivity to ph of the exchange for the amide protons of residues 4 through 14 is attributable to the drastic reduction in [OH−] in the micellar interior, leading to a decreased exchange rate. The A15-G16 segment represents a transition between these two regimes. The data are thus consistent with the notion that the peptide inserts into the hydrophobic core of a membrane-like structure and the A15-G16 dipeptide is located at the micellar-aqueous boundary.