The putative fusion peptide (23 amino acid residues) of the HIV-1 virus is markedly hydrophobic, implying poor solubility in water. On the other hand, it gives rise to a pronounced degree of surface activity. We present here a series of pertinent surface pressure data at an air/water interface. Applying a novel evaluation procedure based on thermodynamic principles, the partitioning equilibrium between the monolayer phase and the bulk aqueous moiety could be determined in spite of the rather small concentration in the latter surroundings. The results are quantitatively discussed in terms of the chemical potential and related thermodynamic properties of the interfacial state. It is in particular shown that beyond a surface pressure of about 15 mN/m the monolayer-associated peptide undergoes a transition to a structure having a smaller effective cross-section (apparently owing to a change of orientation). We propose that our present approach may be extended to peptide/lipid mixtures and should as well be more generally applicable to other poorly soluble surfactants.