The partitioning of membrane-active oligopeptides into membrane interfaces promotes the formation of secondary structure. A quantitative description of the coupling of structure formation to partitioning, which may provide a basis for understanding membrane protein folding and insertion, requires an appropriate free energy scale for partitioning. A complete interfacial hydrophobicity scale that includes the contribution of the peptide bond was therefore determined from the partitioning of two series of small model peptides into the interfaces of neutral (zwitterionic) phospholipid membranes. Aromatic residues are found to be especially favoured at the interface while charged residues, and the peptide bond, are disfavoured about equally. Reduction of the high cost of partitioning the peptide bond through hydrogen bonding may be important in the promotion of structure formation in the membrane interface.