Non-covalent complexes for a tannin—protein interaction model have been analyzed by mass spectrometry. The model is the polyphenol penta-O-galloyl-d-glucopyranose (PGG), a representative member of the hydrolysable tannin family and the nonapeptide hormone bradykinin (BDK). This is the first observation by electrospray ionisation mass spectrometry of non-covalent complexes for a tannin—protein interaction model. The technique should prove to be a powerful tool of investigation in this field.