Proton magnetic resonance spectra at 270 MHz of polymyxin B, a cationic oligopeptide antibiotic, show the influence of the inorganic counteranion present in solution. Hydrogen‐deuterium exchange rates for the amide protons are of two types, depending on whether the anion is monovalent or polyvalent. Polyvalent anions catalyse the acid‐catalysed reaction more than the monovalent anions. The structure in solution was monitored using the proton signals of the amides, the phenylalanine aromatic protons, and the leucine methyl and γ‐CH protons in several polymyxin salts. The temperature coefficients of the chemical shifts of the N‐H protons are used to identify two β turns in the cyclic ring of polymyxin B. The variation in chemical shift of the N‐H protons, the aromatic protons and the leucine protons are correlated with anionic size and electronegativity. Copyright © 1978, Wiley Blackwell. All rights reserved