AbstractFast atom bombardment mass spectrometry (FAB MS) and fast atom bombardment mass spectrometry‐mass spectrometry (FAB MS/MS) were used to study the monovalent glycoside polyether antibiotics maduramicin α, β and δ and the maduramicin α salts, their derivatives and degradation products. Also, representative compounds from three major classes of polyether antibiotics were studied: the monovalent polyethers, nigericin and monensin A, the divalent polyether lasalocid A and the monovalent glycoside polyethers septamycin, BL580 δ, etheromycin and carriomycin. The respective FAB fragment and decomposition ions were correlated with the known structures. The FAB spectra of all the polyethers contained metal‐adduct molecular ions. Protonated molecular ions were absent. All the polyethers having a β‐hemiketal carboxylic acid group produced an abundant ion, often the base peak of the spectra, 62 daltons less than the corresponding metal‐adduct molecular ion. The gas phase mechanism proposed for the formation of this fragment ion is an unusual unimolecular reaction which is initiated by an intramolecular proton transfer from the carboxylic acid to the hydroxy group of the β‐hemiketal, and, then followed by the concerted losses of water and carbon dioxide to produce the corresponding