Tetrapeptides Trp-Aib-Gly-Leu-NH-Ar (Aib: α-aminoisobutyric acid, 2-amino-2-methylpropanoic acid, Arâ =â phenyl or 3,5-dimethylphenyl) were synthesized. The peptides bound quaternary ammonium salts as guests in CDCl3. For every guest, the binding constant K of the peptide host which has a 3,5-dimethylphenyl group was larger than that of the host which has a phenyl group. ROESY analysis of the complex revealed that the N⁺â CH3 groups of the guests were close to the aromatic moieties of the host in the complex. The charge in cation guests, the Ï -basicity of the host, and the turn conformation of the peptides were important factors for the complexation.