The interaction between salivary proline-rich proteins and plant polyphenols (tannins) in the oral cavity and their subsequent precipitation influences the taste, texture and nutritional value of food; it is thought to be responsible for the astringency of many foods and beverages. To investigate the interaction, two-dimensional 1H-NMR studies have been carried out on the binding of a representative polyphenol, pentagalloyl glucose, to two synthetic peptides (19 and 22 residues in length) that are typical of the repeat sequence of mouse salivary proline-rich protein MP5. Intermolecular nuclear Overhauser effects and chemical shift changes show that the main binding sites on the peptides are proline residues together with the preceding amide bond and amino acid. The interaction is principally a hydrophobic association between a galloyl ring and the pyrrolidine ring face containing the C alpha proton, but secondary hydrogen-bonding effects help to stabilise the complex. Very similar interactions are seen for both peptides. The conformation of the peptides remains extended on binding. The chemical shift changes seen for many of the peptide protons can be fitted to a simple binding curve with dissociation constant of around 40 mM, but some protons show evidence of cooperative binding involving several galloyl groups. Higher concentrations of pentagalloyl glucose lead to a reduced off-rate from the complex and eventual precipitation which implies that precipitation is caused by a kinetic competition between aggregation and dissociation of the complex.